Hello!
It might be interesting to run through World Size Check - Docs - The Forge to give us some information.
Though, this is usually caused by Accelereation not being properly on, you could try the last couple of steps in How to fix/turn on your browser's hardware acceleration
though, lastly it might be an ISP issue. in this case it usually solves in a day, but it doesn’t seem to be the case. Could you try to ask them to remove IPv6, or try to change your server region?
Another interest might be to ask them to join an new empty world.