Hi @K9cousin   

Thanks for reaching out to us. 

Actually, when you set up AX55, there is no need to power down AX20. And after you set up AX55, just connect AX20 to AX55 with Ethernet cable. 

Not knowing your previous settings on AX20, but if you want AX55 and AX20 to work seamlessly, it is suggested to change the AX20 wireless network name to the same with the AX55.And you can find how to change the wireless settings of AP in this guide: How to configure the TP-Link wireless router as Access Point?

If you have other concerns, please feel free to let us know. 

Like support already wrote, no need to turn the old one off. I setup an AX55 and had an AX21 already running. No issues. Both are setup as AP's as I already have a Cisco router in front of the ISP. Like he mentioned use the same SSID on both AP's and I turned on Smart Connect so there is only one SSID broadcast in the house. It sort of a poor mans mesh network now. Works better than expected. The only things the TP link devices are missing is a way to only allow WPA3 devices to connect. Also, like other vendors WiFi do now, if you cannot lock the security to one type, they inform you of what security protocol is being used from each client right on their web interface. We all need a way to specify just WPA3 so you can weed out the older devices you might not want on your network anymore. It must be a challenge living in an apartment with many others being able to sniff your wifi traffic or potentially using WPA2 Krack tools.