No network with the hermes pda

I bought the hermes which is unlocked and has the tnt rom installed,i did a hard reset then put my o2 sim in and it wont connect to the network and if i do get a signal i cant ring out receive calls or send texts and when i turn it of then back on it does the same thing over again,the longest its took is about 36 hours to register with the network then die again
I would be greatfull for any help as ive searched google for ages for help and ive only had the thing for four days