NextDNS stops working on MacOSX

I've been using NextDNS for a couple of months now, mostly without any issues. But recently, I encountered 2 occasions where NextDNS wouldn't resolve DNS queries anymore at all. 

I don't know what happened, but today, I notice while I had this issue with NextDNS that when I enabled my VPN (Shimo app), I couldn't route traffic to outside my VPN like I normally can but traffic to my 10.0.0.0/8 ranges works fine and is reachable. I only redirect 10.0.0.0/8 to my VPN so other traffic should flow fine. I didn't make any config changes to my VPN for the record, and this worked before without any problems.

However, I do think these 2 issues are related in some way or another. After I rebooted my device, both NextDNS and my VPN setup were working as before/expected. 

Does anyone get any advice on what I can do to debug the situation the next time it occurs? In the past, I did run the diag tool, but that didn't yield any useful information afaik.

I don't have any knowledge of how VPN and NextDNS tie together so perhaps someone more knowledgeable has some valuable resources to share which I can use to read up a little on how these connect?

Thanks, Niels