In the DNS over HTTPS section of the user manual for the Balance 20x (8.3.0), it states that the addresses it uses for Quad9 are 9.9.9.9 and 142.112.112.112. I can’t find this 142.112.112.112 address associated with Quad9 anywhere other than in this manual. I believe that this is an error, and the secondary address is supposed to be 149.112.112.112, as that is the one listed on Quad9’s site.
I hope that this is just a typo in the manual and that Peplink routers set up to use Quad9 are not sending DNS requests to an invalid secondary server.