Intermittent DNS resolution issues

I have been using NextDNS for a few weeks and overall I am quiet happy with the service. Innovative service and great web interface to configure all you need with great dashboard.

Unfortunately a week or so ago I was forced to move back to Cloudflare due to intermittent DNS resolution issues. 

My setup is that I have a FritzBox (7590) and I enter the NextDNS DNS servers as a DNS-over-TLS servers there. All the devices in my home network get the FritzBox as DNS (for local name resolution) and the FritzBox will forward to NextDNS DoT servers when required.

Usually everything works perfectly fine but a couple of times a day everything just stops for about 10 seconds until it's back to normal again. When I say stop it means website won't load, games stop working, VoIP calls drop and I get kick-out of online meetings. 

This happens randomly, couple of times a day, with no specific timeframe in between them. I tried couple of times when this happens to just run a dig to for example google.com and it won't resolve. When it's over all is back to normal and works as before.

I am pretty sure that this has nothing to do with my Wifi or ISP (Telekom) because I didn't have these issues before NextDNS and as I said as of a week or so ago changing my DNS in the FritzBox back to Cloudflares DoT resolver it didn't happen again once.

I really would like to continue using NextDNS, paid for a yearly subscription as well, but with this kind of issues I just can't. Is there is anything that I could check or change to make this work please let me know.