Your issues sound exactly like ones I had a while back. I have a Netgear R6400 and it performs excellent. No problems with any of my devices. I don’t use remote connect but I left it turned on just in case. When I first installed the router several years ago I manually setup port forwarding. Everything worked great. Then several days later Tablo told us that ports needed setup. But they were setup, and they were working. This went on for a while and became a pain so I just turned remote connect off. Months later I thought I’d try again to set it up. The instant I turned on remote connect in Tablo, the Router and Tablo setup the ports for me. After that it worked great with no problems. All this time UPnP was turned on.
So my conclusion was that if I want to manually setup the ports then I should turn off UPnP. If I want to keep UPnP turned on then let it setup the ports automatically and first clear out anything I may have manually setup previously. That’s what UPnP is for anyway.
I don’t like having UPnP turned on so currently it is off and I’m not using remote connect. I always planned to manually setup the ports some day (with UPnP off) but sorry I have not tried, yet.