This is an update to my post of November 10 and November 17. My CradlePoint COR-IBR1700 router has been working fine for the past couple of weeks. Initially the load balancing seemed rocky, but after a couple of weeks it seems to be working fine. It rotates through the ethernet ports every few hours. So for a couple of hours I am on eth0, then for a few hours on eth2, then for a few hours on eth3. (eth1 is the local LAN; the others are set to WAN. eth1,2,3,4 are user configurable LAN or WAN, eth0 is WAN).
I am 12 days into my monthly cycle and have 13GB on each of my 3 units, so I am happy with the IBR1700's ability to load balance. It is overkill for this application, but I had it sitting in the closet gathering dust and I am much happier with it than I was with the Peplink load balancing unit. The only drawback of CradlePoint is that they don't distribute firmware updates anymore unless you buy a subscription. So if you are considering Cradlepoint for anything, just be aware of this.
I am still getting very slow download speeds (0.1Mbps download, 2Mbps upload. Yes, you read that correctly). But at least availability has been 100% for the past several weeks. I called Sprint, they wanted to send me a MagicBox, but otherwise don't have a solution. Sigh...
I ordered a range of tiny coax cables for my HTC 5G hub units (thank you belusnecropolis for the pointer to LTEfix). These might attach to the circuit board and I will try external antennas once they arrive. At this point, I'm just stabbing in the dark about which connectors are the antenna connectors (see my previous photo of the HTC 5G hub circuit board).