a single APN for web, sms and mms

[kqlnut]

I thank you for the information but in fact the problem comes from this my carrier requires that 2 APN be activated simultaneously one for internet access and the other for MMS. It seems that this is not possible for the moment with the pinephone: impossibility hardware or software? I do not know if it can evolve and where I can go on the forum to find out.

It's not a hardware limitation, but it seems like support for this use case is not yet implemented and development is slow because not everybody has this scenario where they could develop and test appropriately. See this bug report. Maybe you could even help out.
You can also check out chatty's preferences for entering APN details for MMS.