How can I connect multiple routers to a modem that only supports one router?

Question

My modem only supports one WAN connection going to the router all the other ports on the modem have been blocked somehow. I already contacted my ISP and asked them about why the Ethernet cable wasn't working and they said that the modem they provide only supports one WAN port connection.

So I was wondering if I got a Ethernet splitter, like for example the TP-Link Ethernet Splitter (Image below), and connect my two routers to the Ethernet splitter would that work, and if it did, are there any drawbacks?

Extra Information

1. My 1st Router is a router provided by my ISP

2. My 2nd Router is a TP-LINK Archer C9

3. The cable connecting the modem to my second router is a CAT. 7 Ethernet cable

Answer 1

A common layout for consumers wanting to use their own hardware is to do the following:

Modem <==> ISPRouter <==> OwnRouter

You would then turn off features on the ISP router that would interfere with your service e.g wireless, DHCP and so on.

Your isp may support a configuration like this:

Modem <==> OwnRouter

But you would have to inquire with their support and see if they allow a customers own router to be connected and used.

Some ISP devices come with a "pass through mode" or modem only mode, which makes them a dumb relay.

If your ISP router does not support this form of configuration officially, you will likely be stuck with your router connected to a 100mbit port, not a problem if your internet speed is below that however. Some ISP devices (The virgin media home hub for example) Have a dedicated port for what they call "Modem mode" where the superhub performs the actions of a modem, and turns off all other features. Your own router can then perform the tasks you wish.