Please check the supported format with at^syscfgex=?, maybe it's different from the classic E3372.
Here it is:
at^syscfgex=?
^SYSCFGEX: ("00","01","02","03","99"),((2000000400380,"GSM900/GSM1800/WCDMA BCVIII/WCDMA BCI"),(280000,"GSM850/GSM1900"),(3fffffff,"All bands")),(0-2),(0-4),((80800c5,"LTE BC1/LTE BC3/LTE BC7/LTE BC8/LTE BC20/LTE BC28"),(7fffffffffffffff,"All bands"))
OK
Well, looks the same, so you can try:
AT^SYSCFGEX?
AT^SYSCFGEX="99",3FFFFFFF,2,4,44,,
AT^SYSCFGEX?
It accepted the command and changed the bands to B3 and B7 (as confirmed by luci-app-modemband) but then it went belly up, see below, it continued running those commands for longer than I pasted here.
This time the interface had "Network device is not present" and a restart didn't fix it, I had to unplug the modem and back. Now it's back to default bands.
AT^SYSCFGEX?
^SYSCFGEX: "03",2000000680380,1,2,80800C5
OK
AT^SYSCFGEX="99",3FFFFFFF,2,4,44,,
OK
AT^SYSCFGEX?
^SYSCFGEX: "03",2000000680380,1,2,44
OK
AT^NDISDUP=1,0
ERROR
AT
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OK
Huawei Technologies Co.,Ltd.
OK
AT+M
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OK
ATM
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OK
ATGMI
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OK
AT
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OK
CG
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OK
+CGMI
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OK
AT+CGMI
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OK
That's the key )
So the script should work as well. What you see is comgt or another app talking to the modem at the same time. You can try changing the bandmask from 44 to any other value like the initial one, but then exit the terminal immediately and see how it goes.
That's good, I thought that the error with AT^NDISDUP=1,0 meant bad news.
You meant like below? It hasn't changed the bands and the modem has stayed up, but I've probably misunderstood what you meant by "the initial one".
Terminal ready
AT^SYSCFGEX?
^SYSCFGEX: "03",2000000680380,1,2,80800C5
OK
AT^SYSCFGEX="99",3FFFFFFF,2,4,80800C5,,
OK
AT^SYSCFGEX?
^SYSCFGEX: "03",2000000680380,1,2,80800C5
OK
Terminating...
Thanks for using picocom
Edit: tried again the original AT^SYSCFGEX="99",3FFFFFFF,2,4,44,, and exited immediately, the bands were changed and the device didn't disappear this time but the virtual interfaces (v4+v6) still wouldn't recover until a restart of the NCM interface, which brought back the default bands (same as was happening using the luci app).
Edit 2: I'm going to bed now, with good hopes that this can work 
I tried the same SIM on a ZTE MF286D and the bands selection works fine so the network side of things is okay, while on the E3372h-320 the bands just won't stick.. if I don't restart the NCM interface the link doesn't re-establsh, if I do the modem goes back to its default bands.
I tried to change the 44 with a few other numbers but I either got an error or the same outcome, no link then back to default bands.
Any other ideas guys?
i hope that you can add a luci-app to choose the network type: 3g or 4g lte