Thanks for the analysis. It makes a lot of sense that an IPv4 client can’t access the Internet via IPv6 gateway.
Should the device then do 4-to-6 magic? I vote for no, but providing an IPv6 address instead - with proper IPv4 and/or IPv6 routes given.
Hi, on my Xperia 10 III the hotspot works (with O2) and VoLTE enabled.
I agree with you. 4-to-6 voodoo where also not my favorit. But a hotspot with IPv4 and/or IPv6 support, depending on the mobile internet connection established, would be nice to see and one step ahead of Android.
Meanwhile I was able to fix the issue for myself and here is the summary of my experiences so far:
In my opinion, the fact that internet sharing only works with an ipv4 mobile data connection is not the actual problem.
By the way, Android also only relies on an existing IPv4 mobile data connection for Internet sharing.
But this behavior must be known so that the possible underlying problem can be identified.
In my case, the original problem was, that an IPv4 mobile data connection could not be established, which caused the following side effects I observed:
(The first two problems also occur if you normally can connect to a mobile IPv4 data connection, but your APN protocol is set to IPv6.)
For some, the missing IPv4 mobile connection might be the real reason why certain things related to mobile data are not working as expected. So maybe it’s worth paying attention to.
If there are one or more of the above mentioned problems then I recommend to check first if there is an IPv4 mobile data connection.
If not, then one should look into the APN settings if protocol is set to IPv6.
Is this the case then set it to DUAL.
If it is already set to DUAL, there might be a similar problem I had. Then a look into this bug report may be helpful:(IPv4 mobile data connection not possible)
Today i successfully used the hotspot with the 10 ii and VoLTE enabled. Using a German Vodafone SIM card, I had no problems (I did not check with congstar (Telekom) yet)
Can I add a quick “me too” to this list? I’ve just tried using the hotspot functionality on my Xperia 10 III and have tried a number of devices connecting to its hotspot to no avail. I’m on Three (UK network) and have two sim cards with them, one a PAYG and one a Pay Monthly. Neither seem to allow me to hotspot correctly. Both sim cards allow the hotspot correctly in my Cosmo Communicator running Android so I guess it’s not a problem with the sim/provider.
I have set the APN settings protocol to DUAL (in fact they were already set as such) and have also tried the other two options just in case. I’m still unable to get hotspot functionality no matter what I set it to (Have also tried turning off and on VoLTE as a test to no avail).
If anyone has this working with Three please let me know!
EDIT: I changed the APN protocol to ‘IP’ and restarted the phone and it now seems to be working - hopefully that helps others!
Let me add another “me too”. I am experiencing exactly the same issues as describes by the original poster. (Xperia 10 III device, Sailfish OS 4.4.0.68, SIM from Congstar (German Telekom)). Mobile data access does not work at all with APN protocol set to “IP” with this provider.
Quick me too on this, it has worked in the past, but after a trip to Germany stopped working even after returning to Sweden. Phone (10 III 4.4.072) has connection, hotspot is connectable, but no internet on the laptop (Manjaro 5.19.16).
EDIT:
Changing the protocol from dual to IP and a reboot seems to work.
Maybe this is the same issue as the roaming problem?
My hotspot works fine in.Germany with Vodafone Germany and dual ip v4 and v6.
Roaming did not work in Poland until I changed to ipv4.
Back again although hotspot is still working.
Setting your data access for mobile networks to ip solves some issues.
Yet another “mee too”! Again with the telekom-reseller “congster” on a 10 iii, no hotspot-internet-sharing possible, but the mobile-data-connection on the phone itself works fine 
The same contract and SIM-Card worked fine in my old XA2 with the same SFOS 4.04… though.
I use my hotspot daily. Often, when I start the hotspot, I can tether but no internet connection. My workaround is:
It almost always connects after I do this.
Thanks for the information, but doesn’t work for me.
I have no problem at all using my Xperia 10 III as hotspot. Tested with Android 9 handy and Linux laptop. Vanha Rauma 4.4.0.72.
You are a so right! I forgot I also set “Mobile network / Data access point / Protocol” to “IP”. Not Dual or IPv6.
Unfortunately that doesn’t work for me, respectively I couldn’t est it: My provider (german provider: Congstar) does not seem to accept “IP” as protokoll: In dual-mode the phone connects to the Internet (though: does not handle over the connection to the linked linux-computer), in IP-mode the phone doesn’t connect to the internet itself 
I cannot be the only person with this problem, any further hints ???
Use sim slot 2
ipv4 is well supported… Not in sim slot1 (for me and other people wirh orange france provider)
Hi, Xperia 10 III user here. I am also having german ja-mobil (t-mobile/congstar) in germany. In Slot 1 i only get internet connection with “dual” IP does not connect - so same issue here. With “dual” VoLTE does work, but the hotspot does not.
Using Slot 1 hotspot is working, VoLTE did not work, and Internet connection sometimes works - not shure if dual or IP, testet both… Maybe IPv4 mobile data connection not possible - #56 by cartron does help, cause its related?!
The 2nd Paragraph you speak of Slot2, don’t your?
Blockquote
Using Slot 1 hotspot is working, VoLTE did not work, and Internet connection sometimes works - not shure if dual or IP, testet both… Maybe IPv4 mobile data connection not possible - #56 by cartron does help, cause its related?!
Sony Xperia 10III user here. On 4.5.0.19 ja mobil (t-mobile de/congstar reseller) hotspot now working with protocol dual - which did not work before.
have a look at Testing CLAT for IPv6-only mobile networks which seems to have resolved the issue with the last update