Could be that the MMS settings on your phone are incorrect, but equally it may be a known problem where some Android phones turn off mobile data when connected to wifi, and because picture messages require a mobile data connection to send. This is known to affect some phones and not others, so some Samsung models, my own Xiaomi. This does not appear to be related to the mobile network you're on.
If this known problem is your issue, work-arounds include:
1) Swapping your phone for one that doesn't have this problem. This may be expensive or impracticable.
2) Turn off wifi before sending a MMS message (if you try and send one with wifi enabled, it will fail after protracted retries, and when you turn off wifi it won't attempt to resend the message even if you try)
3) Go into the phone's developer settings and set mobile data to "always active". In theory the phone should still prefer wifi for data use, but keep an eye on your mobile account to track mobile data use.
