Version 2.0 supports DD-WRT, and will make a great upgrade in place of the router's stock firmware. Read the full review of the Trendnet TEW-812DRU AC1750 Dual Band Wireless Router.
The AsusWRT firmware supports something called “Dual WAN” - an option to have two Internet channels connected. Apart from the dedicated Ethernet WAN port, you can either plug in a 3G USB modem, or use one of the 4 LAN ports for a second WAN. In my case, I connected the ADSL modem to LAN. Dual WAN works in two modes. Routing 1 LAN + 2 WLANs + 2 WANs (failover) with DD-WRT on Multiple WANs on DD-WRT look here and maybe this will help as well. Dual WAN fallover: here. The tutorials look very detailed and documented. Using them or slight variances should get you what you want. I use DD-WRT myself and it has been a great router for me. No internet with WAN IP : DDWRT
Refurbished: Asus Certified RT-N66R Dual-Band Wireless-N900 Gigabit Router, DD-WRT Open Source Support. Wireless Data Rates: Up to 900Mbps Frequency Band: 2.4 GHz / 5 GHz WAN Ports: 1 x 10/100/1000M Interface: 2 x USB 2.0 Model #: RT-N66R Item #: N82E16833320154 Return Policy: Standard Return Policy $59.99 –
Connect the backup WAN cable to the ethernet port marked as "4" and reboot. More to come . Automation of connection switching with email notification Web interface to monitor and switch connection See also . Dual-WAN for simple round-robin load equalization; Dual WAN with failover How to configure Dual WAN on AsusWRT routers - Leonid Shevtsov The AsusWRT firmware supports something called “Dual WAN” - an option to have two Internet channels connected. Apart from the dedicated Ethernet WAN port, you can either plug in a 3G USB modem, or use one of the 4 LAN ports for a second WAN. In my case, I connected the ADSL modem to LAN. Dual WAN works in two modes.
Dual WAN with failover - DD-WRT Wiki
Dec 15, 2017 How To Set Up Your Dual-WAN Router - SmallNetBuilder