I have placed these two nmos transistors in order to isolate them. Now do the guard rings need to be connected, and if so, where?

@Mohammad Tahmid Hassan For isolated pwell regions using deep nwell with surrounding nwell guard rings, the nwell should be connected to a level higher than the isolated pwell regions and the psubstrate. Normally, it is connected to the same voltage as the nwell used with pmos.

You mean to VDD is it?

Yes.