Op have you ever used a VPN to access the site? Most times issues like this can arise when you use vpn especially free VPNs.
Issue like this arise when someone's IP are connected to other accounts. It might not be the use of VPN. If it is the use of VPN that Rollbit do such a thing, it is better you avoid the site.
having the same IP or using a VPN should not be the only reason a casino uses to accuse a user of multi accounting. i'm sure they look at other data, like where deposits/withdrawals are coming from and going, device fingerprint, etc...
if an accusation is merely based on having the same IP, it's pretty weak and unfair.