Makes sense. Unfortunately, when I lock it at 5GHz (using the instructions you provided), the SP3 doesn't detect the MWDA any more. I had to revert to auto.
Ugh. Ok, that will teach me to not try something myself first. To test the 2.4/5GHz scenario, you would have to reconfigure the Apple router for separate SSID's. At least for testing. You can switch back after testing. it would be good to know whether or not you have this issue when you are explicitly connected at 5GHz.
EDIT: it may be that the MWDA only operates at 2.4 GHz, and if you use 5GHz connecting to the router, you reduce the congestion on the 2.4 band. I'm looking for more info.
Bottom line, it's interference on the 2.4 side that is causing issues.