Sand tnt Cannons absolutely work, raiding is fully functional too.
You were not able to raid the base probably because their shield was active. Another reason could be your tech being incompatible with the server JAR.
You need specific cannons on the server (However, I think your issue was the first one). You can check the shield of a faction by either /f grace <faction> OR /f who <faction>. Also make sure your cannons actually work and are capable of blowing watered walls.
Good luck with raiding!