@joker47man You may have exceeded the max *wifi* client limit of your router (it's different from the max LAN client limit).  Try restarting your router next time you loose one, if they come back it's a router problem and you may need to restart it periodically.  If they still drop you may need to add a WAP with a different SSID and move some of them over.  Router chipsets usually have a wifi client limit, often 20-50 but router manufactures seldom publish that limit.  Smarter wifi devices (laptops, phones, etc) will simply reconnect next time they need to send (bumping another client), some Iot devices struggle as they don't always realize they've lost the connection.

  @joker47man Ok, so it sounds like the wifi limit that would cause an issue is the 100 concurrent.

Since they worked before you know it must be your new router, so of course it's a case of eliminating potential issues till it's sorted.

Always start with a reboot of the router as they're notorious for picking up gremlins the longer they run, doing this helps avoid misleading symptoms.  A power cycle or restart of any kasa devices that don't reconnect, then wait to see if any are lost.  If it takes longer then you know that's the issue...

Next try the most conservative settings on your 2.4GHz band - I don't see that you've mentioned the channel width - I use 20MHz, they don't need much bandwidth anyway.  N only if you can.  See if there are any QoS, fairness, or extra security settings that might need to be changed.

A DHCP problem implies the DHCP server on your router has an issue, it's not a problem from the kasas'.  Could be dropped renewal packets.

Use the kasa app to check the wifi signal of the problematic devices a few times each, compare the average strength to the ones that work - if they're all low that's likely the problem.

If you have multiple APs the kasa's may be connecting to an AP too far away, they don't always see the closest.  The easiest solution is to give your APs different SSIDs and force the kasa devices onto the closest.  If you can't, you might be able to use your old router in AP mode or as a separate subnet just for your kasa devices (route it through your main router).

Good luck!