5G home internet keeps dropping

OK you are troubleshooting intermittent connectivity. Let's break this down just a little. Your solution with the ASUS and the Pi-Hole network-level advertisement and internet tracker blocker opens a whole plethora of variables. The Pi-Hole is a creative, interesting software solution. It acts as a DNS server but what big boy DNS server upstream does it use? I would probably test with pings to the Google, Quad9 and CloudFlare DNS servers and pick the one with the lowest latency to point the Pi-Hole at to feed it what it needs. Given the Pi-Hole sits in the middle maybe in the tools folder there are some useful trinkets. Even without clever tools in the Pi-Hole I would speculate you have  a Linux client or two, I do. Using the terminal and "netstat" there are various options for how to get the facts and build what we know vs. what we think. Taking packet captures and doing analysis on the functions of sessions is another option. Using Wireshark or taking a tcpdump can provide important clues. I don't have a Pi-Hole but I can download it and install it on one of my Linux clients to tinker with. Since I have not used the Pi-Hole software I am not the best resource on that solution but I find it very interesting. If you suspect an incompatibility between the Nokia router and another router/device then look at the communication across these devices. The answers are quite possibly in the frames/packets, session information, from the TCP/IP stack etc… There could be some issue that looks like an incompatibility but is really just a fundamental bug. Con datos you will have the silver bullet to put the wolf down. 

On a side note. I observed first hand that if I leave a HTTP session with the Nokia router too long, I have not timed this, the web UI of the Nokia router will report no network connection. I used my MacBook with CAT 6 direct to LAN 1 on the router and a Linux client with a wireless connection. I opened three terminal windows and ran simultaneous pings to the Google DNS, Quad9, and CloudFlare DNS servers. All the pings went right on through and the router was really connected to the tower but the web UI reported incorrect information. I closed the tab and opened a new session to the Nokia router and taadaa the router reported the real facts. Sometimes it takes multiple forks to get proper confirmation and it takes time. I am sure you are putting considerable time into your solution given it is not a garden variety solution. You stated the ASUS was reporting the internet down but how do we quantify that? It seems similar to when I observed, on multiple occasions, the web UI of the router HTTP server being unresponsive. Sure I should have taken packet captures to confirm which side of the session was being mute but it was not that important to me as I know how to work around it. It may be how the Nokia router caches information and when the session fails it acts like Jack Oneall in Stargate, "I got nothin." I could be accused of that at times. :-)

I have spent most of my time investigating the network disconnects with ONLY the T-Mobile Nokia router and my local clients. I know it seems rather rudimentary but it has worked for me quite well. I did 3rd level network troubleshooting for +20 years on enterprise networks so since I retired I have taken a little more relaxed approach to my current network solution. I currently have over 10 days of uninterrupted uptime on my network with just the Nokia router so the KISS principle does have some merit. There are some interesting posts out on the web but don't let all that chatter take you too far down the rabbit hole. Look at the basics before jumping to the more complex thoughts. Taking packet captures will provide hard data points so you know what you know and not think what you know. My father always told me if you know the why you can figure out the how. It works for me. Around 20 years ago a network escalation engineer told me, "I don't want to know what you think. I want to know what you know." I listened to Dave and became a much better engineer. I ended up taking his seat in the office in CA and did the job for +20 years. The key was to open my mind, eat humble pie, and dig for facts. I was also fortunate to have landed in a position with some very creative team members who all became great friends. 

The T-Mobile "router/gateway" may not be God's gift to mankind but it is not a "trashcan". Don't let others put a negative spin in your head. Think for yourself and determine if you really think it is a trashcan or not. I have found it very limiting for visibility to the actual network operations but in the end with careful investigation working around its limitations I managed to obtain a stable solution. The problems I had with these same issues were not with the Nokia router but with the cellular equipment on the tower. By working with the T-Mobile support engineers I was able to see the problem through. It did take a couple of weeks before things became stable but they fixed the problems. I get 10X the download and upload speeds I got in CA for less money using the T-Mobile home internet solution. It was a huge win for me. If it ain't broke don't fix it. If you can get some hard data and I can help out I will try. Please just call it a router and keep it neutral. Best wishes!