KVD21 gateway dropping LAN connection or cell connection?

That's not the issue here. There is not much you can do with KVD21 as far as router management. The only thing that most users will able to do is to disable its WiFi function which can reduce some of the KVD21 resources. As for external router configuration, I have tried in both AP and router modes. Both have its own advantage and disadvantages. With AP mode, you losing most of external router management ability, other than better WiFi coverage, you are pretty much depend on KVD21's routing capabilities and performance. My experiences were slight lower on the speed range and better ping times. With external router configured as router mode, you can utilize all the router function to configure it with your specific network needs. Many users concerned double NAT situation since you can not make KVD21 as a bridge, but if you are not a among user or with some heavy port forwarding requirements, double NAT has little affecting for internet accessing. With my experience with ASUS Zenwifi Pro XT12 mesh router configured in router mode, my speed consistently in upper end of speed range with slightly longer ping. The other benefit of double NAT, it is more secure for my home network since I have no way to knowing what or how KVD21 capable in terms of router security.