Issue with iSCPC Modem

We are having some problem in iscpc Link.

FTP is working, But HTTP is not working. Customer has no firewall or proxy. Just connected LAN cable directly to Modem.  TCP and UDP compression was enabled on the modem . when we disable both , it worked fine and later we enabled again and everything was ok . But now the problem arise again that HTTP is still too slow or not working.

No firewall or Proxies in customer side.

Please let me know any suggestion.