If you got a certificate from your ISP (internet service provider?) i think you did not get a ‘self signed’ certificate.
On a ‘self signed’ certificate the issuer and the owner name is the same.
If you take a look on the ssl certificate of the forum you got the following information:
Issuer: R3 (from organisation “Let’s Encrypt”)
Owner: forum.sailfishos.org
If your certificate did not have the same issuer and owner, you have to go up the certificate chain to the ‘root’ certificate (this is a ‘self signed’ certificate) and put this one to the phones certificate store.
You have to trust the ‘root’ certificate (the first one of the certificate chain) .
On the example above you have:
… … forum.sailfishos.org certificate ← ssl certificate
… … … Issuer: R3 (organisation “Let’s Encrypt”)
… … … Owner: forum.sailfishos.org
… R3 certificate ← intermedia certificate
… … Issuer: ISRG Root X1 (organisation “Internet Security Research Group”)
… … Owner: R3 (from organisation “Let’s Encrypt”)
ISRG Root X1 certificate ← this is a ‘self signed’ certificate (also known as ‘root’ certificate
… Issuer: ISRG Root X1 (organisation “Internet Security Research Group”)
… Owner: ISRG Root X1 (organisation “Internet Security Research Group”)
So at this example here you have to get the ‘ISRG Root X1’ certificate and put it on the certificate store of the phone.
I hope this help