If you reboot the router, can you then log in from one of the currently "forbidden" devices, and if so can you then log in from any other device?
It might tell you whether the problem is a limitation to one device only or just the first one to succeed… but I can't think why it should do either
Re Mick's idea of one session at a time- I've had that, it avoids the possibility of conflicting configuration changes being imposed. I always thought it was a good idea, but on one router (forget which) I had to wait a few minutes after logging out before I could log in again, even from the same machine. That
was a pain at times.