Makes sense, your IP is blocked so when you switch over to your phone carrier's IP, you are not blocked.
It has happened to me before. It is the board blocking the IP. This can happen for a few different reasons:
1.) Case of incorrect identity - your IP is blamed for something that someone else did.
2.) Your IP was previously occupied by someone who was doing something in conflict with the board's policy and when you grabbed that IP, you became banhammered. Slim chance though as it would be someone on the same ISP subnet that you took the IP from.
3.) If you're on a VPN, same logic, someone else could have had that same VPN IP, did some uncool shit, and got banhammered.
If you are on a VPN, try a geographically different server. Or, if you're not on a VPN, just phonefag for a while until the ban expires.
I a bugling like a bull elk right now. Next step is to piss on my carpet and roll around in it. LOL!
Glad to be of help.