your mobile provider sim has 2 IDs (mcc / mnc) to match an “apn” config that is then used to setup the mms address. There’s tons of threads on mms if you follow the topic tag. It’d be worthwhile to compare the APN settings that are chosen by the device vs. what the provider is suggesting in their help pages.
If the apn config is proper and mms work, there are still some conditions on size that can play a role in delivery (there was one fp5+mms match on that).