5G home internet keeps dropping

I had another thought or two about troubleshooting this. You have clients upon the ASUS and still have LAN 2 for leverage. The Roku test was informative but what about connecting a small gigabit switch to LAN 2 and using a client or two multi-homed in effect. You could use the investigation client(s) with wireless to the ASUS and have an Ethernet connection to a switch on LAN 2. If the client has issues hitting the internet through the ASUS then disable the wireless and enable the Ethernet port and check results. Maybe take a client and the Nokia router with the 2.5 Ghz wireless on a different SSID and look at results from that angle as well. El Asus ac68u has four Ethernet ports as well as the 802.11ac so take advantage of the physical ports on the ASUS as well. Test and verify. If you have a client with multiple Ethernet interfaces and plenty of resources stand up some virtual machines as well. Linux  would provide yet another data point and physical Ethernet LAN ports tend to just work with Linux. Lots of tools in the Linux environment to play with. Use different clients, Apple, Windows, Linux, Android, Raspberry PI clients. OK maybe I am making assumptions but today it is not uncommon to have all of the above. 

Another parallel investigation you can do is leverage your cell phone and if it is an Apple iPhone put it into field test mode. See if it communicates to the same tower as the Nokia router by confirming both report the same PCI values for the tower signals. Android phones have applications for tower location so yet another option. If you have not yet used cellmapper.net to validate the location of the tower your router connects to i highly recommend doing so. It is simple enough to use and very informative. What cellular channels are you linking to? How strong and clean are the signals? Do the cellular signals bounce/change from one channel to another? Do any of the local devices record errors or drops? Profile the behavior in detail. Focus on any device that can be influential and stands out but keep an open mind on other actors in the path.

Use the web searches to get ideas but focus on the facts. The values and behaviors you can confirm. Don't overlook all the tools you can put to work. Keep notes and analyze the operation in a systematic periodic manner. 

The objective: Determine with more certainty where the problem resides. Look at the physical layer before you go up the stack.