@Super-WiFi 

You don't want to increase hops... it's better to link to a root or single hop AP, even at -86dBm.   I would look at positioning to improve the new AP's access to a better node.

You also cannot be the 4th hop off a 3rd hop AP for performance reasons, I believe Omada limits its mesh to max 3 hops.   I used to design meshed networks and my rule was max 3 hops, max 3 neighbours, so believe me when I tell you that you don't want to be 4 hops away from a wired node, not only will your performance be terrible, but you will degrade the performance for everybody upstream of you.

  I saw your note about 15 APs, and one of them refusing to join as a wired AP.  The problem is most likely cabling (again) or failing that the backend VLAN/routing.  You should verify that it is actually wirelessly linked and not just trust the wifi icon displayed.  

I can confirm that if I disconnect one of my wired APs, it will come back linked via mesh...so no explicit config is required for the device to operate in either mode although wired always takes precedence.