@Mcmclau 

Please check if the Wireless Schedule is enabled on the AX20 or not, if you enable it, the router will stop broadcasting the Wi-Fi, and you will lose the wireless connection on the phone:

https://www.tp-link.com/support/faq/2596/

As for the speed slowing down, when that happens, you can observe the WAN and LAN port status of the router here:

https://www.tp-link.com/support/faq/2265/

You can determine if the corresponding WAN and LAN ports is connected and negotiation speed is correct, while the issue is in effect.

If the problem is with the WAN or LAN port connection then you may try changing the Ethernet cable with another one (Cat 5e/Cat 6) and test again.

@Mcmclau 

Please check if there is any setting on the AX20 that will turn off the network, such as Wireless Schedule, when you are connected to the network.

Meanwhile, please ensure you are running the latest firmware on the AX20. If it is not the latest, update it first, then confirm again:

https://www.tp-link.com/support/download/archer-ax20/#Firmware

Guidance for updating router firmware manually:

https://www.tp-link.com/support/faq/2139/

It is also suggested resetting the router to factory defaults if you are not sure whether there is any special setting that may cause the issue, then reconfigure it from scratch to confirm.