GL-AR750S Extend Mixed Network

I’m trying to use a GL-AR750S to connect a wired device wirelessly to my home network. My network has two meshed Unifi AP’s, and the network I’m trying to connect to is both 2.4 and 5GHz. The 750S can see both the 2.4 and 5GHz networks, but refuses to connect to either as an extender, saying that the connection timed out. If I create a wireless network on either AP, and pick 2.4 or 5GHz, then I can connect. I’d rather not have a dedicated network for this device, and I’m not willing to change my otherwise working wifi network for one stubborn device. Is there some trick to this, or is connecting to a mixed network not possible?