a single APN for web, sms and mms

[landnull]

To avoid the need for 2 APN, I kept the one that allowed me to have access to the internet and I deleted the other one (the one that allows to send MMS), then I set chatty as follows:
MMSC=that of the MMSAPN (the 2nd APN that I had deleted)
APN=web access
PROXY=that of the MMSAPN (the 2nd APN that I had deleted)
The information was given in the forum of my provider.
Today it works and I can send and receive MMS. 

For this to work, it is obviously necessary that access to mobile data is activated: be careful sometimes after a restart it remains checked when it is not.
I thank all the developers for their efforts and perseverance.

So for someone to use chatty, Both who are texting must be sign up for chatty correct?

No.