Teltonika RUTX11 EMEA LTE / 4G Router | WLAN 802.11ac Dual Band 2.4GHz + 5GHz WLAN, Dual SIM Slot, LTE Cat6-Standard

LTE Cat6 Router with transfer speeds up to 300 Mbps, Gigabit Ethernet, WLAN 802.11ac with transfer speeds up to 867 MBit/s, Bluetooth v4.2 LE

First I will explain what is Load balancing and WAN failover.
Load balancing lets users create rules that divide traffic between different interfaces.
Backup WAN or WAN Failover is a function that allows you to back up your primary connection in case it goes down.
In your case, I recommend using WAN Failover. In the RUTX11 router settings, you can set the internet that comes with cable to be the main WAN and the mobile LTE internet as the WAN failover. When you lose the internet access through the cable (WAN port), the WAN failover is instantly activated and the device uses the Internet over LTE.

The RUTX11 routers have one modem, so only one SIM card can work at a time. If you want two LTE networks, then as you mentioned, you need two routers. The data flow of both routers can be combined using Load Balancing. More information about Load Balancing:

https://wiki.teltonika.lt/view/Load_Balancing