Help connecting smart plugs

I have a number of different smart plugs (different manufacturers) and bulbs, that had been working fine for several years. Most were installed using my older 2.4 router. Now I have the newer one (arris TG1682G). Originally all of the plugs transitioned over just fine with the new router, since I kept the same name/pwd, but recently one plug started acting up and when I tried to reset and reconnect, it would no longer connect to the manufacturer's app. Then I tried to add two new smart bulbs (different manufacturer) and nothing I could do could get them connected, even using the AP mode. I know that the smart devices need to be added to a 2.4 wifi, and once added should work on a 5, but I can't get that far!  I've had people on one help group tell me that I need to somehow set my router to 2.4 long enough to register the devices and have had others tell me that no, the smart device will automatically connect to the right band. (That seems to be true since I also bought some new plugs from yet another manufacturer that require 2.4 and their app automatically worked within a few seconds).

Anyhow... HELP. If I need to somehow force channels(bands? whatever) to switch, can someone point me towards a step by step instructions? (And how to get back to "normal" afterwards). I have never tinkered with my router, other than to periodically power cycle it (and no, that didn't help either).