I followed these excellent and simple instructions to set up a WiFi repeater on Buster: [Access point as WiFi router/repeater with additional WiFi-dongle However when I rebooted the eth0 access was gone, the Macbook will not connect on ethernet. I can still ssh by WiFi but I want eth0 access for speed and reliability. Also I want to route internet from eth0 to wlan1 and vice versa, while keeping the WiFi AP on wlan0. How can I do this?
With the setup you have used it is right that the wired interface has gone. It is simply not configured because it is not used. To use it in addition to the access point the best way is to bridge it with interface wlan0. How to do it you can look at Using an additional WiFi dongle to create WAP and switch.