I have a SIM on COSMOTE in Greece and having this weird issue where occasionally the cellular modem goes into a state where it continuously flaps. If I watch the internet tab in the console I can see it lose signal and reacquire. All signal strengths are listed as good or excellent, 4G LTE. If I set a continuous ping running it usually hits almost exactly 50%, 11 pings up 11 pings down. If I reboot 4-5 times sometimes eventually it will stick. I also travel a bit with the router and it doesn't seem to happen everywhere I go. I've restored from factory defaults, I'm on the newest firmware, disabled network acceleration, and disabled internet status tracking. I don't really know how to troubleshoot this. Any ideas?
Update the modem module firmware to the latest - I don’t mean the router firmware !
Then try to lock the band of the tower.
Is this a known issue and a known fix?
Which firmware?
Where do I get it?
How do I not brick my expensive router? 
All steps are detailed here:
Make sure to download this file to the router:
https://fw.gl-inet.com/tools/quectel_tool/QFirehose-mtk7981a-sha256-c0b944
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I tried firmware RM520NGLAAR03A03M4G_01.201.01.201 as per the post and was getting about 3.5% packet loss no matter where I put the router and the ul/dl seemed capped at 10M for some reason. I also had some recurring errors in the kernel log, but forgot to make note of them. It was almost unusable for me.
I browsed the gl site for the open directory with other firmwares and found RM520NGLAAR03A04M4G_01.200.01.200 which I can't see referenced anywhere or hosted anywhere else. It had approximately the same timestamp as RM520NGLAAR03A03M4G_01.201.01.201 so I took a chance and flashed it. The 3.5% packet loss is gone and ul/dl is now at 20M which was what it was before. Also no flapping (yet) so I think the original issue I had is fixed. I'll keep checking.
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Have never heard of this version ! But hey thanks for sharing it !
I am glad that you have got it working.
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