Here's a quick post showing how I solved an issue with an unreliable LTE backup connection. Depending on your hardware, firmware & UI version YMMV.

The problem

It appears that Huawei E3372 frequently disconnects from the LTE network for no apparent reason. It wouldn't be much of a problem if it managed to quickly and reliably reconnect, but that isn't the case it would seem. Whenever that happens packets get lost and websites don't load properly, and overall internet experience is well below acceptable. After some googling it turned out that it might be caused by E3372 automatically disconnecting from the network, but fortunately enough, automatic disconnects when idling seem to be a configurable feature [sic] in the HiLink version of the modem. Less fortunately though, the default interval is set to 5 minutes, and obviously needs to be changed, potentially even disabled altogether. Let's see what the WebUI has to offer in this regard:

Oh...