Client side lag

[Innuiyasha]

So, I've recently made a tekkit server and there is a fair bit of lag rendering the server near unplayable.

This problem really confuses me because of one main thing: the lag only exists while I'm using my router. If I unplug my router and plug directly into the modem there is not a second of lag. All of my users have been experiencing lots of lag no matter their proximity.

Is there anything I can do to try and solve this problem? I can't get everyone to directly connect so that's not an option.

Is there maybe a problem some kind of client-side port that needs forwarding?