How to isolate a WAP (WiFi router) from the internet, but not the LAN?

// This is my first foray into actually asking a question on stack-exchange, if I have erred in location or protocol please let me know!

Basic Problem:

I am trying to set up a Wifi router in an specific area, so that when connected I can access some utilities/services that are available on our LAN, but at the same time not access the internet. These utilities/services are hosted by a server on the LAN, and I want them to also be available to the rest of the (internet-connected) LAN. As a final note, I would like to preserve whatever form of name resolution (DNS) I end up adopting within the LAN.

Context/Details:

An example of one of the services is Kiwix, which currently hosts Wikipedia some other machining references. Another is WikiJS.

I currently plan on using an old Linksys WRT54GL with stock firmware as the WAP, however if DD-WRT or different hardware would be a better choice that is not an obstacle (read: VLAN?).

Research/Current Work:

I tried hunting around the web, and here, but was unsuccessful in either finding anything terribly similar or describing my problem.

That said, I gather there may be many ways to get the job done. The simplest, which I have not yet tried, would seem to be connection an existing (internet connected) router/switch into one of the LAN ports of the router, leaving the WAN/Internet port empty. This will be my first attempt, but I figured I would seek confirmation as well.

Alternatively, I know there are potential router firmware options that would facilitate behavior like this, perhaps involving a local DNS server?

And “lastly”, we have a pfSense firewall at the edge of the LAN, and my understanding is that this could easily be utilized to restrict access to the internet for a particular WAP.

If anyone could offer confirmation as to whether one of these, or another approach would solve my problem, thank you.