The unsupported protocol is likely due to the connection being lost.
It has been reported by many others, that the Sierra MC7455 modem in particular doesn't play well with Sprint sim's, moreover CDMA/LTE service tends to be more finicky than GSM/LTE (AT&T, Tmobile) during initial network connection/reconnection.
Try swapping out both modems with a Sierra EM7565/7511 or some other brand.
Also the way the router handles internal vs external modems, it could have slightly different setup timings or perhaps glitchy, so a good test would be to attach both modems externally in separate enclosures, to see how the system behaves.
If none of the above suggestions seem to help, then as a last resort you may want to consider replacing the Sprint sim with another carrier.