Hi @Mescubasteve ,
Judging from your original report and assuming your cabin receives mains electricity from the house, I would recommend that you switch to Devolo powerline adapters with a WiFi access point in the cabin. Other brands are available.
I do not get the impression you would enjoy the details of getting your second router to play well with your main router such as differentiating between WAN and LAN ports, or understanding IPv4 subnets, and the function of DHCP, broadcast domains and routing.
My above suggestion will spare you this pain. 🤓
I myself have advised other users on here that the minimal configuration approach is to use the WAN port on the second router in a double-NAT configuration and I use this configuration myself. This is typically the best choice when there is a new router which has become the main router (such as when the ISP has changed) because the old router will essentially work as before in most cases.
If the second router has no WAN port, you would go for a LAN to LAN configuration as others have advised above. To get this to work successfully, you would need to disable DHCP server on the second router to avoid a clash partitioning your broadcast domain into two segments which don't talk to each other, which it likely why you see the current problems.
The tricky bit with LAN to LAN is maintaining access to the second router's management function and to do this you would need to understand a little about subnets and IP addresses. At this point I personally would be tempted to go for the bespoke tidy option which eliminates the long cable and gives you 1.2 Gbps in each direction and which might exceed gigabit Ethernet.
I also have the option of running a long Ethernet cable the entire length of my property. I even have a cable in excess of 15 m from my uni-days, I've simply made the choice to use Devolo powerline adapters since before 2009 instead.
Good luck! I would be interested to hear how you get on learning about home networking. 😁
An additional benefit I find with a powerline adapter access point is that it attaches to a power socket so it takes up zero footprint and doesn't even block the socket as a full-blown router would when used as a wireless/WiFi access point.
If you do want to go on the adventure of turning a second router into an access point, the pictures on wikihow should give you comfort. https://www.wikihow.com/Connect-Two-Routers
It would be slightly tidier to purchase an WiFi access point instead of repurposing a router which in principle is a device to perform routing such as between networks/subnets.
