Broken QoS on MR600
The QoS feature on this router is absolutely terrible.
Instead of using any popular algorithm for QoS what you are doing is reserving a percentage of bandwidth for high/medium and low priority users/protocols. To do this, You ask for the expected bandwidth completely ignoring the fact that this is actually a LTE router and in 99% cases, People won't be getting constant amount of bandwidth throughout the day unlike for example a Fiber or ADSL connection.
So, We have no way to know how much bandwidth the router will be able to pull from a cell tower at any given point in time and as such, We can't enter the expected bandwidth in the router.
If we enter a smaller value than the actual bandwidth, speed on the devices gets capped compared to the speed they could be getting.
If we enter a higher value than what is actually available, QoS just doesn't do it's job all that well.
Ideally, You should be prioritizing traffic from a machine/port by following any of the popular QoS algorithm but what you seem to be doing here looks like a complete mess.