Group messaging and file sending problems on Galaxy s8 with T-mobile

[Iang12]

I recently switched from iPhone and AT&T to tmobile and galaxy s8 and I've been having a few problems... First, I can't send or receive messages to or from group chats of over 20 people. This is a big problem as I am in quite a few large group chats. Also, for some reason, when my friends with iPhones try to text files or pictures, they say "not delivered" and I don't get them. Same with me sending files to them. How can I fix these? Also, I upgraded to the 200 gb option of Samsung cloud and I have 56gb of stuff on my phone, yet Samsung cloid says only 16 gigabytes of the 200 is being used. This is concerning because I want all my stuff backed up in case something happens and I have to restore. Please help find a solution to these problems as I love Android but am still trying to make the transition.