Verizon may not help on this. The SIM I used works just fine in smartphone and only get 0.5Mbps on routers. I didn’t all what I can do including changing TTL and IMEI.
As the router is not Verizon certified and they will not do anything to help. On the contrary they will ask you not to use on a uncertified device.
So yes - Verizon was limiting the SIM. Originally, they registered it in store with a tablet they had. I think they eventually (a whole year later) must have changed something. I had to upgrade my plan so that “hot spotting” was not throttled at 600 kbps.