R03 includes regulatory fixes that somehow makes the performance unstable while R01 branch is the "generic" global firmware
R03 has better performance for me. I get much better upload speeds. So I guess it really depends on your tower,location and personal situation doesn't it?
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Most probably it's operator-related.
I upgraded to the once suggested RM520NGLAAR03A03M4G_01.201.01.201 and it was really a nightmare (disconnections, band aggregation problems) until I tried RM520NGLAAR03A04M4G_01.202.01.202 and that finally gave me stability again.
Now, out of curiosity, I tried RM520NGLAAR01A08M4G_01.205.01.205 and it's as stable as RM520NGLAAR03A04M4G_01.202.01.202 for me, go figure....
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I agree with this. I see a lot of variability between carriers such that there is not a single firmware that runs best in all scenarios. It takes some testing to see what works best for your situation.
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As @Max3 has stated, I think it is best to stick with either of the latest firmware visions only.
RM520NGLAAR01A08M4G_01.205.01.205 or RM520NGLAAR03A04M4G_01.202.01.202
They have been updated for a reason. As a side note, TMO has been doing a lot more testing with their 5G SA mode Country wide as of late so this in itself can be a major contributing factor for some users in certain areas.
But what is difference between R01 and R03 does it matter if I am in Europe?
In terms of stability, R1 and R3 are stable, but R1 and R3 belong to different baseline versions and have the following differences:
Compared with R1, R3 has a relatively large adjustment in the code framework, so after upgrading directly from R1 to R3, due to the incompatibility of the software framework, some key configuration parameters related to the network will not take effect, and some operators' networks may be disconnected and unstable
Conclusion: Therefore, it is recommended not to upgrade from R1 to R3 directly, and notify you of the upgrade after Quectel provides a compatible upgrade solution
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@Javon_ma Thanks a lot. What about going R3 to R1 (both latest) ... I happen use R3 but probably do not need adjustments from that branch. Would you discourage doing so?
@tasinofan As mentioned earlier, if R01 is upgraded to R03, some operators will have network disconnection and instability, so if your current R03 is stable, you do not need to downgrade to R01
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@Javon_ma : Thank you. I understand, but I wonder, why calling going R01 a downgrade ? That sounds like missing something or going back in time. But both trains move forward I suppose and then my question was also about : should I consider train R01 without adjustments if it does all what I, most probably, need (in Europe) and avoid relatively large R3 extras? I can say, my requirements and needs are modest 
@tasinofan There might be a misunderstanding. The downgrade is only regarding the version number naming. For the R01 you mentioned is stable in Europe, but now you have upgraded to R03. You can try using it for a while first. If there are any network disconnection issues, you can revert to the R01 version, and that should be fine.Thanks！
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That sounds like a good strategy ! I really appreciate your expert reaction!
Thank you so much.
This has been my experience as well with the exact same modem firmware.
I updated today to RM520NGLAAR01A08M4G_01.205.01.205 by downloading the zip from the link in: GitHub - 4IceG/RM520N-GL: Quectel RM520N-GL is 5G IoT module specially optimized for IoT/eMBB applications.
And then drag'n'dropped it to the local module upgrade page.
I had issue where in every 2 hours the modem stopped working by either giving the SIM card not registered or showing a spinning icon for the cellular:
Hopefully after this update it will work properly.