Just to round things off, I wanted to point out two guides with pictures on accomplishing both options mentioned above. They both mention LAN-to-LAN and LAN-to-WAN (either in the text, pictures, or headings).
https://www.wikihow.com/Connect-Two-Routers
https://www.wikihow.com/Cascade-Routers
I was quite surprised to find picture-based guides as the configuration is relatively simple but the pictures widen the audience to beyond the audience I would have expected.
Although I LAN-to-WAN my desktop/large-footprint routers, I recently fell victim to buying more than I need with Devolo Magic-2 WiFi Next and LAN Triple from Amazon Warehouse for a total of about GBP 135 (it was to remove an apparent 300 Mbps bottleneck which I imagined and was not present in the replaced dLAN adapters).
My opinion is that technology companies make a lot of profit from people buying more than they need so right-sizing purchases and re-using existing hardware are always good things. 😀
Quick mention that TP-Link N300 seems a pretty reasonable choice. WiFi extender so it effectively uses WiFi as backhaul instead of powerlines when using powerline adapters. An advantage over powerline adapters is not requiring a powerline adapter at each end. The disadvantage is the bandwidth will be limited by the WiFi link and the extender will need to receive a good signal. If one of the routers could have functioned in WiFi bridge mode, it could have taken the place of the TP-Link N300. On the other hand, a router capable enough to have a WiFi bridge mode would typically be too expensive to waste in such a function (I flashed DD-WRT to a compatible cheap router to get this functionality).
