Extender or Access Point and issues with AIMesh

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I keep having WiFi dead spots in my house and want to get WiFi in my backyard. Using a RT-AX88U as the main router with RT-AX57 set in AP mode on the other side of the house with wired together with cat6. The AX88U buried in a structure wiring panel that is WiFi compatible (plastic). I tried using the AX57 in AiMesh mode but the router would stop functioning completely after sometime, even devices wired to it would not register a connection but works perfectly fine in AP mode. Now I want to add either an extender (RP-AX58) or access point (either EBA63 or another brand POE AX access point) to improve signal strength. I currently and not using the same SSID but would like to use 1 SSID per 2.4g and 5g to help devices roam. Any recommendations on a preference and what could possible be the issue with AiMesh not working on the secound router?