Speed Drops the regains and restart ?

Curious that the behavior only recently started. When the behavior is such that the download have dropped down it would be good to know a couple of things. You have the Nokia gateway as you lovingly refer to it as a trash can. :-) Well, it sort of resembles one but compared to the other two newer ones I am not sure it is so bad myself. Mine has been a good little can so I am not convinced they are all bad. 

Let me get to the point. The Nokia is so simple to get the cellular metrics from and also get the PCI values for the primary and secondary cellular signals that is where we should start. I suggest to take some time and focus on the cellular metrics and record what your gateway reports. On the overview page it reports the cellular signal metrics for both the primary and secondary frequencies. When the speed is up or down record these and compare good to bad. Is there a change is what I am after. Also the same needs to be done with the cellular information on the Status page for the cellular network information for both primary and secondary information. Paying attention to the PCI value for both but mainly for the 5G NR secondary signal source. Does the PCI value change or is it the same. Does the PCI value change after the reboot of the gateway for one or both or either signals after the reboot? Does your gateway receive the n71 or the n41 5G cellular frequency or flip back and forth?

The reason I ask about the PCI, physical cell identifier is that it could be when the speed is so reduced the secondary 5G cellular source is NOT the same as when it was 10X before the degredation. The PCI relates to the cell that the signal source originates from. If you use CellMapper.net to locate the cells that are related to the reported PCI values you can determine exactly where the tower is that has that cell. The 4G LTE and the 5G NR cells might be on the same tower or may not be but if the gateway does see another 5G cellular signal that is stronger than the one it had and it meets the threshold to move then it may lock onto the "new" signal source and have different behavior. The move may or may not make sense but it might happen. It sort of depends upon your location with respect to different cellular signals. 

The speeds you get for download are not very impressive for 5G but well more like say a DSL connection which I have had and it is rather meh… so not trying to be too critical but I would expect better for 5G. With that said the speeds possible can and will depend on multiple variables and if you are out on the edge of the reach/range of the 5G signal source that would explain why it is slower. 

The performance is NOT just about speed. If you have a strong signal that is a quality signal and has low signal to noise ratio then it will perform better. A signal with good strength and poor quality and noise is still probably going to perform bad as noise and a poor quality signal will lead to packet loss and retransmissions due to packet loss or damage. So, what i am saying is the metrics matter and you have the gateway that has those tools easily available to reach and use.

You can combine the observations of the cellular metrics with speed testing with say speedtest.net and fast.com to profile the behavior. I prefer to run speed testing with my MacBook Pro on a wired Ethernet cable vs using the wireless. My MacBook Pro is from 2015 so not 802.11ax capable. Older network adapters will not be able to perform as well as the new ones and can influence the testing. With speedtest.net I suggest to download the application, create an account, and then configure the preferred server as a baseline to test against. Test against a number of test servers to begin with and find one or two that seem to provide positive results. Pick one you like and use it as a reference. The more consistent your testing is the better the profile. Once you have the account you have a historical record of the testing so you can have a clear record of how the behavior is or was. If you make screen clips of the information and keep up with things you can build a good profile and really know what to expect. 

I know it seems a bit excessive but it helps. Knowing where the tower is that has the cellular source really helps to get the gateway in a better placement. Don't just settle for a couple of bars and it works. Try rotating the gateway clockwise or counter clockwise and see if the signal wash over one side of the can or the other raises the metric values and improves the operation. I have my gateway in a window in the basement with exposure somewhat to a tower 5.3 miles north of our home and I spin it around so that the back side, where the Ethernet cables plug in are facing in. Sure it is ugly but it improves the signal wash over the 5G antennas on the "front" side of the gateway by -3 to -5 dBm for RSRP and usually improves RSRQ and the SINR by a db or two. You may understand the metrics so please forgive me for being verbose on the matter. I will include the chart I use for reference.

Hopefully the information will help you tune it up so you can get a better, stronger signal. Your downloads are much lower than I would hope for given it is 5G but once you have a clear picture of what the cellular metrics are telling you I am sure might be able to gain some improvement. I say this but IF he distance to the tower puts you on the edge of the cell reach for 5G the only recourse for that is to use/add an external antenna. The external antennas are not cheap but they can do wonders for improving the signals. If both the 4G and 5G signals originate on the same tower establishing a 2X2 or 4X4 MIMO antenna, depending on the 5G frequency, can improve things. 

You asked for ideas. OK, so there you go. Probably more than you wanted to know but it is cellular and not a cable solution so it takes more effort and work.