Internet is making my gaming lag badly, it use to be good then all of a sudden we can only have 1 console at a time, i am unsure on what to do

Question

I have a lot of lag when gaming and can only have 1 console connected at a time even then it still lags, we have a normal router and a gaming router and the gaming router is connected to the normal router but we are still having massive lagging and disconnection issues, i am not a tec genius and have no idea what i am doing when it comes to how to fix the problem.