Hi,
Sorry to hear that. If you have discord, have you tried using that to join chats? A lot of university societies and subjects have discord groups.
Don't worry too much about struggling to join group chats before uni starts, I personally didn't join any until freshers week and you don't miss out on anything. A lot of other people I know didn't either so a lot of people will be in the same boat as you. Everyone is pretty willing to talk to new people at the start of the year, especially during freshers and there will be many opportunities to meet people in your college/on your course during welcome week.
Hope this helps,
Becky (Lancaster University Student Ambassador)