The most sensible thing to do in this kind of situation is to call your operator & try to get to the bottom of this issue.
But since you asked, these could be the possible issues.
1) Wrong APN :- Check your APN settings & look for any typo error. Call customer care to know if APN settings has been updated in the recent past.
2) Baseband and/or RIL issue :- It could be possible that matching Baseband & RIL are not flashed, assuming that device is rooted & has been tinkered with.
3) Missing AWS Frequency Band Altogether :- Model no. P990 refers to two different models Optimus Star & Optimus Speed. The former doesn't has support for AWS band. So it could be possible that your device is the former, not the latter.