there is no such thing as a ADSL AP that I have ever heard of… What is the make and model number of the device?
yes I would recommend the unifi line.. The new AC lite model is $89.. Or again any wifi router you have laying around be it ethernet or dsl based, if it has a modem its actually a gateway (dsl, adsl, etc..) Still can be used as AP... Just disable its dhcp server, change its LAN IP to be on whatever network you going to connect it to. Connect it to your network with one of its lan ports.. access the IP you gave it and setup the wifi how you want.
When using a wifi router as AP you don't use its WAN port, since its not going to be routing or natting anything. So unless the device has mode to bridge its wan to its lan the use of that port is lost. But what does it matter if just using it as wireless AP.. You can also leverage the remaining lan ports if you want for wired connections to your network.
But a decent AP is going to be POE and designed to mount in the ceiling for better coverage.. Your typical wifi routers are rarely poe and not really good for design for proper placement for good wifi coverage. While some can hang on a wall, etc.. since they need power source they make it more difficult to place in proper area for best coverage normally.
Do yourself a favor and get a real AP, maybe a few of them if area your wanting to cover warrants it. The unifi line use a software based controller, or you can buy a little key to run the controller on, or run it on any pc you have or vm, you can run it in the cloud on aws, etc. etc.. The controller is going to give you lots of insight into clients connectivity, bandwidth used, etc.. etc.. There are even some guides on running the controller software on pfsense itself - but I would not recommend that.
You don't have to use the controller if you don't want to either if your not using its captive portal feature, etc. They are even releasing a APP to quick configure the AP with if you don't even want to run the controller for initial setup of the AP.
https://www.ubnt.com/unifi/unifi-ap-ac-lite/