Well... I guess I'm not quite visualizing the situation...

So just to make sure I understand correctly:

1) Do the Ethernet cables in your house, all lead to one central place, like a closet, or maybe some point in your living-room perhaps?

So for example, do you have a bunch of wall ports in your house, in which the Ethernet cables run through the walls, and then all lead to a central point...

and at that central point you might just have a bunch of Ethernet cables coming out of the wall (maybe with labels on them, like "bedroom" or "kitchen" or whatever)?

Or maybe at that central point you might instead have patch-panel of some sort?

2) If yes, then I assume what you are saying, is that if, for example, you plug your T-Mobile router into one of those Ethernet cables at the CENTRAL POINT, such as the cable that leads to the "bedroom" for example, and then you go to the other end of that cable whereever it may be (bedroom for example)...

and then you plug in a computer there, you can then get Internet access, and it works fine?

3) If yes, then that's great so far!

What it means is that we have one working device (T-Mobile Router), and one working cable-connection leading to another point in the house.

So that's a good start.

But what you need to do next is TEST every single other cable connection at that central point, that leads to other parts of the house.

So for example, you need to plug the T-mobile router into the next wire or port at the central-point (such as the "kitchen" connection), and then move your computer to the kitchen in, plug it into the Ethernet port there, and test that connection as well... etc...

repeat until you've tested all connections.

4) Once you've tested all connections with just a computer, and the T-Mobile router, it then becomes time to plug your switch into that central point, where all the cables of the house lead to.

So for example, if you have a bunch of cables coming out of the wall at that central point, you want to plug all those cables into the different ports on your switch.

Or if all the cables running through the house, terminate at a central patch-panel, then you'll need a bunch of small Ethernet patch cables, running from the ports on the patch panel, into the individual ports on your switch.

5) NEXT... you will then connect the switch's WAN port to your T-Mobile Router, and then that should give your entire home connectivity to the Internet.

If it is a true switch you have, then each cable running from each room of your home, into the switch will be it's own separate isolated "collision domain", so that it's data does not collide with data travelling along other Ethernet cables in the home at the same time.

So the switch keeps them mostly isolated, assuming it is a TRUE SWITCH, and not just a "HUB".

6) But of course you need to make sure your switch is PROPERLY CONFIGURED.

Sometimes you might get lucky, and the switch will simply work out of the box. If it's a consumer grade, simple switch, then it should just work out of the box, hopefully.

But sometimes you need to make configuration adjustments.

And that can be a bit complicated if you've never done it before.

For that we would need to know what type of switch it is, etc...

SIDE NOTE:

If you don't want to deal with a switch, you can try buying a cheap hub with a lot of ports on it. If that works great, then just stick with the cheap hub!

But the problems with hubs, instead of switches, is that with a hub, all devices on the network can see data-traffic from the other devices, so the central lines get crowded.

A switch on the other hand, isolates each cable/wire into it's own collision domain, so that the devices don't have to see all the crowded chaotic regular data traffic of the other devices.

But sometimes for a home environment, a cheap hub with lots of ports is enough and works just fine.

ALTERNATIVELY... instead of a switch or hub, you can buy a router that has several ports on it. You then put the router into "bridge mode", and then plug the router into your T-Mobile router.

So you end up with a T-Mobile router connected to a second router. (That second router is put into "bridge mode".). That second router then uses all of its Ethernet cable ports to connect to the rest of the house.

Once that second router is in "bridge mode" it acts like a switch.

So basically, if you buy a second router, what you are doing is ditching your current switch (which you can't figure out how to operate, if it seems too complicated), and then replacing your switch with that second router (which you know how it works, since most home routers are simple)...

And then again, that second router now becomes your new switch (since you'll put it into bridge mode).