Based on your conversation with the support agent, it seems they’re quite confident that you logged in from a device that other users have also used.
You mentioned accessing Rollbit from your school’s PCs, which could explain it.
Additionally, you said you used your girlfriend’s laptop, does she also have a Rollbit account?
Unfortunately, proving that you don’t have multiple accounts can be very challenging.
It is true that such a situation would be very difficult for OP to prove his innocence but I think the casino should be able to differentiate between a public in the sense that there have been many logins through a particular IP which seems to be a public IP or system and that of OP login. If it happens that it is OP first time of accesing his account through that IP they should be considerate and also be able to see the difference between OP regular login IP and that which made the blocked OP account.