Interesting. Having a similar fleet of RUT955, doing similar work like you, but using shell-scripts and LUA AND running newest "official" openwrt in a custom built firmware image, I would like to know about the batch number of your new RUTs, just for verification. We have RUTs from various batch numbers, most of them batch 13 or batch 14, running in production in the USA, using Verizon for WWAN, being fallback for WAN or WIFIWAN.
Or do you think, it is a problem of a new firmware version, running on your new RUTs ? Then, to verify, you should flash old (running) firmware to the new RUTs. Do they work now ?