Search the internet for "<carrier name> MMS settings" and save the page or write down the values.
On your 635 go to Settings > cellular + SIM > SIM settings > edit MMS APN and make sure the values match the ones in the search.
If you still need help, join the site and reply in this thread with the carrier name and country you are in so we can help match the right values for you or track any carrier related oddities to advise you of.