Hugely variable speeds

That is curious. So, with the "normal" speeds the RSRP reflects a weaker signal power but speed improves. The lack of SINR values does not help. Do you know 100% sure the signal in each case is on the same frequency with the same cell origination for the signals? It sort of looks like the router is making a transition from one source to another and back. Identify the PCI values for the 4G & 5G signals  in each case and see if they are changing. If the gateway is switching back and forth between cells that might account for the inconsistent behavior. It is much easier to get the info with the Nokia GW vs the Arcadyan GW so knowing which is in service might help. It might be possible there is work being done on the tower(s) in the area and that is causing the disruptions or maybe there are load issues. It is hard to tell with just the current data. 

Does the change take place fast and at frequent intervals or does the service change with delay and then also do so at frequent intervals? I know you state in a matter of minutes so this is a very frequent behavior that just persists or what? 

Using the PCI values you should locate the tower(s) with cellmapper.net and determine distance to the tower(s) around you and which one(s) the PCI, physical cell identifiers originate the signal from.