R7000 WIFI drops often on V1.0.11.126_10.2.112, sometimes cannot connect

Firstly, I personally wouldn't downgrade to v.42. It's a four year old + firmware that will undoubtedly lay you wide open to potential security issues. Some will shrug and say 'Maybe, maybe not, don't worry about it' but you makes your choice and you takes your chances.

I'm assuming as you mention VPN that you've had your router chipped? I use a VPN myself, but it's a standalone app, not something I've added to the firmware. If I have any misunderstanding of your set up, please accept my ignorance. I certainly wouldn't try to upgrade/downgrade your firmware with your VPN connected. If I have my VPN connected and I try to access my R7000 browser GUI, it just won't let me log in.

I have to admit, I still struggle to understand why some people have issues upgrading firmware. I've upgraded six times since I bought my router from new, never had an issue with the actual upgrade. I never do a factory reset and the router auto reboots anyway once the firmware has uploaded. I suppose though you could just do a proper factory reset and then start afresh, but what that will do for your VPN, I have no idea.

I highly advise you don't attempt to upgrade the firmware over WiFi, use a stable wired connection. I also disable WiFi in the router prior to upgrading the firmware, but that's a personal quirk for me.

Make sure your MTU packet size is set for your particular ISP. It should auto detect, but sometimes it doesn't get it right. Also, if your country allows it, transmit power on both 2.4 and 5ghz can be altered in the browser GUI, mine is set at 100 percent on both. Also make sure your using the rest of correct settings for your ISP, PPPoE/PPPoA etc.

Other stuff. I have disabled UPnP and auto update. I've also enabled HTTPS only. I've also changed my ISP DNS to opendns servers. Check your NTP server too and set if for your locale.