As I said in my last post, my GF on Android and her brother on Ios (both much more expensive phones than my GS5) have exactly the same issues. It’s a provider thing in some areas.
We were also in Denmark on the Moen’s Klint (chalk cliffs) recently and I noted that I would get 0 coverage until I took a real risk of leaning out to sea 
In any case, I did ‘nothing’, drove back to civilization (cough) and had 4G in the next town.
I have the vague impression that all persons commenting on German mindset here are themselves German…
And still when I leave Germany and go to Romania, I have the feeling that I have a new phone … Internet connection works smoothly and fast. In Schwarzwald, I always have limited or no connection at all.
I could imagine that if you come from abroad to Germany your experience also would be better than with a local sim card, because under the roaming rule set your phone will always connect to the locally best available network and not just the one of your own provider.
Might be! I use in Germany 1und1 and Vodafone.
Update: problem still remains, but change 4G to 2G comes everytime after call…
Same problem here in the UK (Cambridge Science Park and city center, home, driving…) using a local Talkmobile SIM (with strong signal), using Xperia XA2 Ultra running 4.5.0.24 (Struven ketju), WLAN is off. I have selected Prefer 4G but the phone just insists dropping soon to 2G (sometimes 3G) and it will never go back to 4G automatically.
Switching Mobile data off and on won’t help. Switching to 2G only and then back to Prefer 4G does restore 4G connectivity for a short moment.