How to enable mms in iPhone 7?

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

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

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.