mesh access point

Try connecting using an Ethernet cable, and see if you can adopt it that way.

Once that’s working (assuming you don’t have Ethernet cabling to where you want the other AP (mesh unit) to broadcast, wired is ALWAYS a better choice, even generally if the AP is only partway to the desired location), try unplugging the Ethernet and moving it progressively further away from the Gateway.

There's a limit, around -60dBm or so, where things will fall off pretty fast, in terms of reliable connectivity (you can use a WiFI app on an Android device to test the signal strength at a given location).  Even at <= -60dBm, you're getting half-signal, given the wireless backhaul.  So, if you get say 100Mbps wireless throughput at the location of the AP, devices connecting to that AP will get a max of 50Mbps (that's 50 shared too, amongst any/all devices using that AP).  Meshing is okay if it's the only option, but it's far from a perfect solution...