Figured it out! My original password had spaces in it. For some reason, this caused 2.4Ghz to fail with some devices (some worked). By changing the password to one without spaces, the EX2700 hooked right up.
Fwiw, I had zero issues with 5GHz, including with other WiFi extenders.
Is this an official restriction on passwords? I googled around a bit and didn't come up with anything concrete.
