1) your 320U able to connect LTE 4G with direct plugin computer USB or with V2 original stock firmware firmware before ?
if it did, then u can band config it to 4G only,.. under huntsman , under huntsmand -> modem -> network status -> change cellular modem -> select 4G only -> click apply mode change

or direct plugin computer USB port use MDMA to band config it to 4G 1800MHz or 2600MHz or both,..

or you can use AT COMMAND via putty to do so as well

2)  there is QoS management under huntsman -> network -> QoS

Mind if I ask what is your LTE speed on stock firmware?

refer back to your question 2

openwrt QoS will not limit the bandwith of a specific ip/port/service out of the box, the idea is that services/ports/ips with a lower quality will be served after others with a higher priority.  Low priority services will still get the full bandwidth when no one else is requesting bandwidth.

so you can set those 2~3 address to low priority ,..

this is different than IP/Mac Address/Service Hard limit QoS like QoS in stock firmware