Basement Suite Wifi

HI all,

I am not super tech savvy, and I need help figuring out internet for my basement suite tenants.  Looks like I have the Nokia ONT (is that right?) and the NH20A on the first floor, and the Wifi 6 boost upstairs in our main living area.

The basement tenants have been on the guest wifi that I set up on the Boost, but lately it has been really really slow.  I would like a permanent solution, and read on here that some people have bought another router and set up a 2nd wifi network.  Also, it was mentioned that a "bridge" would be needed too (dont know what that is).  I would like to do all that, but have no idea how.  I am able to do some basic networking stuff, so if anyone could provide me with some instructions, that would be fantastic.  Thanks.