A faster service from your ISP and/or additional lines from your ISP with additional routers. If you have very fast service from your ISP then a Mesh system may be a better option also.

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.

Bypass the router and connect the PC direct to the modem and run your speed tests. It may be your ISP or your modem that has the delay in the ping.