You can have what is called a 'Wired repeater station' but will not work AiMesh (not an issue because you don't want that). I normally assign the wired repeater station a static IP on the main router. Static IP should be set outside of DHCP pooled assigned address range.
2.5G network ports, it is my understanding, will only run at this speed, if they are capable, that is both connected NIC support this and the cable used, CAT6 (usally not crossover, may get away with CAT5 on shorter runs 'less than 5m to maybe ?10m?'). Not really confident about CAT 5 distances, stopped using it fair while ago, just off memory thing and less than 100m comes to mind without signal booster.
