as update, the TP-LINK support was quick, they provided me a built where you can specify a Ping or a DNS test, with the new build the failover works much better Firmware Version:1.1.0 0.9.1 v0001.0...

Hello, I used the backup FAQ "How to set up the 3G and 4G backup" https://www.tp-link.com/us/support/faq/2719/ I have my router in - Operation Mode "Wireless Router Mode" -Network/Internet/Static IP...