good point... Could the coverage pattern be different for the guest network? I do not know for sure but would think the guest WIFI would use a different channel than the main WIFI. So I would explore...

The next thing I would try is moving the router, turning the router, move the antennas if moveable, changing the height of the router, use different WIFI settings such as a specific channel instead...

Did you use the same SSID on the Linksys? By chance, are the two devices that will not connect 2.4 or 5 only ghz? Try deleting/forget the network settings on the two devices and then reestablish them.

Does your ISP enforce some type of data cap? Also check your network settings. May need to delete and re-add your network settings on your PC.

Check your guest network options as many TP-Link routers allow scheduling for the guest network but not the main network.

TP-Link site says it does. https://www.tp-link.com/us/home-networking/wifi-router/archer-c1200/