You cannot simply connect a router to another router. Doing so would create IP conflict and crash your network every time.
A preferred way of adding a second router as an extender is to adjust its LAN IP to one other than 192.168.1.1, disable its DHCP server, and establish a LAN port to LAN port connection between the second router and the main router.
My home forum is the Fios one. We have customers using older Verizon routers as extenders when they get a newer one.
Underbus, We definitely understand the conflict with wanting to connect your home router to your existing home network. Just as Cang_Household mentioned, this wouldn't be possible as the LTE router can only work as a standalone connection to our towers. May I ask for the reason you are looking to connect multiple routers to your existing home network? We would like to see if there may be an alternative to what you were trying to accomplish. We will be standing by at your earliest convenience.