Welcome to the forum.
I think we need a bit more detail here to help answer your original question. Which HD2 variant? Which single cellular MAX model? Assume the US, what sort of mobile? Man portable? PAtrol vehicle, police boat, command and control trailer?
The HD2 ‘Classic’ is Firstnet band 14 compatible https://www.peplink.com/products/max-multi-cellular-hd2-router/
You could also look at the MAX Transit Duo https://www.peplink.com/products/max-transit-duo/
The HD2 Dome is also Firstnet B14 Compat. https://www.peplink.com/products/max-multi-cellular-hd2-dome-router/
The MAX Transit Mini is Band 14 and Band 71 Firstnet Ready. https://www.peplink.com/products/transit-series-max-transit-mini/
The choice between them will come down to desired form factor and throughput. Personally I’m a big fan of the Transit Duo LTE-A Pro / CAT12 model as I think it hits a particularly good bang per buck target and its small, versatile yet powerful enough to do just about anything you need it to.
That said the HD2 has POE output on the LAN which is great for mobile CCTV installs, and has separate wifi radios for wifi wan which is better in command and control vehicles.