If I’m picturing the gear right, putting the TP into AP mode would just make it a client of the network that would then serve as your WiFi and the new box could be set up as the router/gateway for both the TP and the other clients formerly plugged into the TP.

Usually, changing the mode from router to AP would keep the LAN side active as an unmanaged switch, and may even add the wan port to it. So if all above holds true go modem, Celeron (opnsense), TP (LAN to LAN) and then plug the remaining Ethernet either into the TP or the other LAN ports on the Celeron box, both should be the same local network.