How to enable mms in iPhone 7?

settings->mobile data->mobile data options->mobile data network->mms

enter the

apn, username, password, mmsc, mms proxy, mms max message size, mms ua prof url

that your carrier use

Settings/Messages Scroll down until you see SMS/MMS. Toggle the switch to MMS on.

That's if it needs to be edited. On supported carriers, you won't be able to do this. On unsupported carriers you can edit out those details.