I recently purchased the MR600 v2 as I occasionally get drop outs on my cable internet connection and want to have a 4G backup. I set the MR600 up in 'Wireless Router' mode and connected the cable modem's LAN port with the LAN4/WAN port on the MR600. The router works normally when I am only using the two wireless radio channels to connect clients. However, when I connect any wired client to any of the other three LAN ports (LAN 1-3) the connection to the cable modem goes down.
 

This is very frustrating because I was planning on using the MR600 essentially as a switch to prioritise the cable internet connection from my cable ISP, but then switch to 4G when that network goes down. I essentially want to turn off both wifi radios and connect my more powerful wifi 6 router to one of the MR600s LAN ports and run the local network on it. 
 

Also just a note, I currently have the 3G/4G backup switch turned off as I have only just started experimenting with this router and have not had a chance to buy a 4G mobile data plan to use as the backup. I can't see how this toggle would change anything though. In 'wireless router' mode the MR600 will prioritise the cable connection going to the LAN4/WAN port. 
 

Is there a setting I have missed that will enable me to connect other wired clients to the MR600 while it is in 'Wireless Router' mode and has a wired connection providing internet to the LAN4/WAN port?

Thank you!