@Heathkit 

I had similar issue on the R605. However, I didn't want to adopt failover.

I had a setup with WAN1 (microwave, stable 80M/10M) and WAN2 (4G, unstable of its nature, with bandwidth up to around 50M/30M).

Whether it was the limitation of the modem (MR6400), how many concurrent sessions it can transmit, or of the LTE operator, I couldn't figure out but for a few devices it could handle internet but with dozens it caused "no internet" issues. So I tried to limit traffic on that WAN2 port, setting up the bandwidth to 1000 (not the lowest possible as you did but should have had a desired effect) on the load balance router.

The result was similarly unsuccessful as yours. The R605 did not respond based on the ratio difference between the two WAN ports that I set up. In my case, it sent too many devices through that port and did not recognize the failed connection, it kept thinking the port was online.

I couldn't figure out why the algorithm doesn't limit the usage at the ration we setup. I mean, if it was the WAN1 port, I could understand that when there is only a single device using the internet, it would always use the first available port. But we intentionally chose the 4G connection as a secondary one, plugging it on a further WAN port.

When you try to monitor the usage, the main problem is that you can't just look at the usage data (how much it sent/receved since it was reset). In your case it can send fewer devices to that port but with larger usage, you can't see the exact connection ratio, just the total usage.

I gave up adopting the 4G connection to our load balance router.

But you can try policy routing some of the connecting devices or just some type of connections exclusively to that WAN3 port.

Mind you that the Link Backup option on these routers will never provide seamless operation. It will always interrupt internet for at least a few seconds before it establishes the backup connection. You can find several videos on youtube showing these results.