There have been a few threads on these forums with people frustrated by the fact that TP-Link doesn't officially support briding mode on its range of cellular routers (see for example here). I was annoyed too. 

In one of the threads here, @Kevin_Z suggested a workaround: if you want to use the cellular router only as a bridge (ie, as a cellular modem) pull down the WiFi, take down the DHCP server, and put its IP addres within the range of the main router.

I wanted to create this thread just to say that: I followed Kevin's recommendations and it worked!

I now have the following setup (all actually using TP-Link gear). It fulfills the objective I was going for when starting these home network upgrades which was a dual ISP / cellular connection that uses the cellular line only for failover / as backup. 

Because this is tough to explain using words, here's the diagram:

My gear Is:

- TP-Link TL MR 100 

- ISP router

Both going into WAN ports on the TL-Link R470T+ load balancing.

From there I actually have one more TP-Link product (an Archer router running in AP mode) to bring out WiFi connectivity. As wel as a switch for all the wired gear on the network.

Sharing this in case it helps anybody also trying to figure this out.