There are plenty of posts saying why the problem appeared, and what the solutions on offer are. Most people don't want to know why realtek chips have DNS issues, every Mede8er I've had suffers from it. I don't know why either. It's buried deep in realteks design/SDK. It drops any network connection where the DNS response is too slow. Most users are just happy to use any DNS workaround that fixes the current DNS problem. They all work by responding quicker to DNS requests. In this case by running a lan based DNS server with an explicit record for the failing DNS request, one which by-passes the Cloudflare added delay.
So are you saying you have a Synology NAS you would like to set up to do it?
P.S. DNS Request...Asking a DNS server for the IP linked to a domain name. In this case api.opensubtitles.org.