New iMac knocking devices off network

I could write a novella about the last 30 hours but here's the relevant.

I have on my Time Capsule-created network Nest, Sonos, Lifx, Hue, Harmony Hub, Echo, Wemo, iPad, iPhone and AppleTV.

I set up my new 2017 iMac Tuesday and I after I did the Wemo, Harmony Hub and Lifx stopped working. Also, the iPad and iPhone told me I had the wrong password for the network. I initially thought this was maybe because I updated to the 7.7.9 firmware on the Capsule but realized the Wemo had stopped working even before that. Power cycling the Capsule seemed to solve the issue with the iPad/iPhone, but not the Wemo or Harmony.

I tried reconnecting both several times and went to countless webpages trying to find troubleshooting tips but nothing worked. The Harmony and Wemo that had been working perfectly for months and months were unusable (the Lifx I had been having on and off problems with forever so I didn't give it much thought).

Then I had the idea of trying to reconnect Wemo and Harmony AFTER shutting down the new iMac. And they connected and worked perfectly UNTIL I restarted the iMac, after which they were both unusable again: lost connection and can't be reconnected.

Two points of note: wifi is OFF on the iMac as it has an ethernet connection; two of the Wemos are at the exact opposite end of the house as the computer.

What's going on here?