In this case we would suggest you to try using different ethernet cable like cat 5E or cat 6 and if still issue persist we need to check the cable signal level on the modem settings or GUI page.

http://arris.force.com/consumers/articles/General_FAQs/G34-G36-Cable-Signal-Levels

what device is it? 

What speeds is it getting? 

You say its lagging, define that more. What speeds its getting, what is its tested latency/ping?

I’ve never seen a hardwired device say it has poor signal. It either works, or it doesn’t with hardwired devices. Are you sure you don’t still have it connected via wifi? Go into the settings on whatever device this is and re-run its network setup to ensure its using the hardwired connection