3310 and 3390 dont work on att 900 mhz

[superman]

I have 3310 3390 8210 which dont log on to att 900 mhz gsm network, but emergency calls work, here in the caribbean, they dont use IMEI checking, any ideas? maybe phones too old? no other 900 mhz network around. I believe the guys here on this island at att dont know what they r doing.