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
My 1st Router is a router provided by my ISP
My 2nd Router is a TP-LINK Archer C9
The cable connecting the modem to my second router is a CAT. 7 Ethernet cable