Wired mesh network?

Currently I have 2 B1300 routers configured as a wifi mesh network. The main router is connected to my ISP router by cable.
I would like to move one of my routers but this one will not be in range of the main B1300 router wifi.
But I have the possibility to connect it to the ISP router by cable as well.
Is it possible to have a cable mesh network through a common router (ISP router)?

You need to get a LAN port of the master node to the WAN port of the slave node either directly or via a switch and you should then have Ethernet backhaul mesh

I can’t connect the 2 B1300 routers by cable. I only have one RJ45 plug in each room that are connected to the ISP router in the garage.
So the main B1300 router already uses the RJ45 plug to connect to the ISP router.