I had the pb, solved for me with the following parameters for MMS Access Point :
Name : Bouygues MMS
APN : mmsbouygtel.com
Protocol : Mixte
Authentication : None
Proxy Address : Leave empty
Proxy Port : Leave empty
MMS address center : http://mms.bouyguestelecom.fr/mms/wapenc
Xperia XA2 Plus
Sailfish 0S 4.1.0.24 (Kvarken)
Peripheral adapter : 0.0.8.19