iMac won't stay with my preferred wifi network

There are several wifi networks in my home, one created by my Velop system and two by my cable modem. Obviously, since I went to the expense of buying the equipment for a mesh network, and have peripherals attached to it, I want my computers to use it. Nevertheless, when my iMac awakens from sleep, it regularly joins the cable network. The iMac sees and gets a good signal from my Velop network (one of the nodes is 2 meters away!) and I can switch to it with no difficulty, but I want it to join automatically. "Automatically join this network" is checked in System Preferences > Networks. None of the several other computers in the house, or my iPhone or iPads, display this behavior.

It is probably irrelevant, but maybe not, that this iMac (21.5", just a few years old and running High Sierra) can be infuriating at times, with exasperating idiosyncrasies, including excruciating slowdowns – when I signed on to post this question, and had to enter a verification code sent to my iPhone, it took FIFTEEN SECONDS for all of the digits to appear in the blanks onscreen, one every couple of seconds, after I'd entered them in quick succession on the keypad – as well as sometimes taking the better part of one minute to launch or quit Mail. And the computer wasn't doing anything else – I had just turned it on. And it refuses to stop asking for 2FA even though I'm not blocking any cookies. Probably unrelated, but just sayin'.