@Sunshine This happens regardless of presence of SIM card. I used polish operator Orange.

BUT I think I found a trigger: restarts happens when router is connected to computer.

When it was alone (no usb connected) - it was up for a long time (I did not notice restart)

When it was powered up from powerbank - also

when I connected it to laptop - it started to restart.

I checked it by

ping -D 192.168.0.1 (-D option adds timestamp in output in linux version of ping ) and saving output to file

then, from analysis of saved file it turned out that router works about  5- 20 minutes, then there is about 2-3 min break (no communication with router) . and cycle repeats.

  @Sunshine 

"may I have the brand and model no. of the computer? and what is the operating system?"

"HP EliteBook 860 16 inch G9 Notebook PC". Strictly speaking firstly I have connected the router through this hub: https://i-tec.pro/en/produkt/u3chargehub16-2 but connected directly to laptop - also restart.

Debian SID.

I might test with Win10 machine but not now.

"When the computer is connected to USB port of the M7450 and when no reboots, is the computer able to access the internet and the M7450 properly?"

I am not sure how to understand your question. If you ask "computer is connected to wifi and has access to internet" then yes, it was no problems.

When you ask if RNDIS interface works - then "I do not know, did not test it" EDIT: yes, also works.

"By the way, when did you purchase the M7450 and was it working fine before?"

Purchased on sunday, got tuesday, 2 days ago, so this is new router.

Another part of this puzzle:

I have connected router to Windows 10 machine and it keept up for about 30min (so I suspected it is ok)

What was more interesting: I pass usb device (router RNDIS) to Windows VM on my linux laptop, and ... it also does not restart for about 30min.

I was completely confused (especially with this VM case) but I noticed, that both windows machines are automatically put up RNDIS interface and took address from router's DHCP.

So I connected this router to laptop, and  brought interface up with addres taken via DHCP from router.

There was no restart for about 1 hour so far.

Stay tuned.

  @Sunshine No. Device restarts when RNDIS interface is not up, regardless of connecting directly or via hub. Last night I left router connected via hub, and interface was up  and router did not restart for 10 hours. When I put interface down router started to restarts.

So now simplest way to reproduce:

1. take debian[2] distribution with ifupdown suite  (no network manager, netplan or similar [1])
2. connect router via usb, and left interface down
3. wait about 10-15 minutes  - router should restart.

[1] with other network suites effect probably will be the same, but I don't use them and I do not know if they are will be trying put interface up automatically, which may hide problems.

[2] of course other distributions also may have this problem, but I do not know how to use them :)

There is another stupid bug: after reboot  device presents

 ATTRS{serial}=="0123456789ABCDEF"

instead of real one.

  @Sunshine Sorry for being imprecise.  By "serial" I meant USB device serial number.