@Poldes-12-booge,

You cannot run two routers on the same network, at least with both of them functioning as routers. There can only be one DHCP server and set of WAN connections for your network, which is provided by the router in its default state. If you would like to use the wireless connectivity from a second router to extend your network, you will need to change the operation mode of the second router to "Access Point Mode." This will make it so that the conflicting settings and features on the second router will be disabled and the signals will be forwarded to the main router.

Your TV most likely was able to connect the first time because it had received its IP address from the Main Router, but when the second router tried to take over the connection, it  assigned a new IP, but this address was unrecognizable by the main router.

tl;dr

You can have more than one physical router in a network, but only one of the routers can be in "Router Mode." Any additional routers must be set up as an Access Point(unless it is a mesh network, which is its own type). Only one Router will be in charge of managing the connections on the network, while the others will operate as signal extenders. And yes, the Ethernet Ports will function the same when in AP mode.
 

PS - If you have additional issue seeing devices across APs, I would recommend looking at the settings on the main router for the network. There are often settings available for AP Isolation that can prevent devices from seeing each other, such as when attempting to cast a video to a TV