Until now, I've used an old router as a switch (i think) to plug different devices in my room, and then I use a PLC to connect this router to the ISP's in the living room.
To do this, I changed the ISPs router IP from 192.168.0.1 to 192.168.1.1 and my other router to 192.168.1.2
I also opened some ports in the ISP router to my devices and assigned some manual DHCP leases.
This works well, but the ISPs router web interface is crappy and limited and sometimes doesn't work, so I want to restore it to .0.1 and make my router .0.2 on one end ant 1.1 on the other and then put all my devices in .1.0/24
Right now in the LEDE router I have this (just restored it)
In your new scenario, each router does its own NAT by default, and the devices behind your LEDE router will get double NAT for their internet access. It mostly works, but is undesirable.
Would it be an option for you to replace the ISP's router with another LEDE router? This is the place where you can see the largest improvement by switching to LEDE. The device in your room could then stay in switch mode (access point), and double NAT is avoided.
@jeff, since @LoLo2207 is using a powerline link between the rooms, I assumed there is considerable wifi signal attenuation between them, and it is not possible to provide wifi coverage in the living room from the LEDE access point in the bedroom.
The device wiki page explains how to configure the switch to change one of the Ethernet ports into a WAN port. https://openwrt.org/toh/comtrend/ar5381u
If there is signal at the router location, it may also be possible to make the device a wifi client of the main router, achieving the WAN link over wifi.
It requires a static route to the LEDE network on the ISP router. And it also needs to NAT traffic from the LEDE router with a destination on the internet. Unfortunately these features aren't available on all routers.
There's a nice explanation of how the switch can be used to create a WAN port where the device has none. The link you have previously already contains the steps how to do it, this article just contains a nicer explanation: https://wiki.openwrt.org/doc/uci/network/switch