Adguard installation question

I would like to install Adguard Home on a secondary router, not my main router. Should the secondary router be set up as a Dumb AP first so it doesn't interfere with the main router, or something else?

Thanks in advance.

Yes, unless you isolate it.

Be aware that, if you do this, you need to make sure your configuration directs DNS clients (or downstream queries) to the instance of AGH sitting on the AP. Simply having AGH running on the AP isn't sufficient on its own to force DNS filtering of clients attached to the AP.