NextDNS DoH connection fails on Keenetic router – TLS connection error

I am experiencing an issue when configuring NextDNS over HTTPS (DoH) on my Keenetic router.

My configured DoH endpoint is:

https://dns.nextdns.io/......

When I add this server, the router repeatedly logs the following errors:

https-dns-proxy:
CURLINFO_SSL_VERIFYRESULT: Error (unable to establish TLS connection)

curl error message: Connection timed out after 3000 milliseconds (28)

The errors occur repeatedly, and the DoH proxy process is eventually terminated. As a result, my internet connectivity is disrupted.

I tested Cloudflare DoH using the endpoint https://cloudflare-dns.com/dns-query. It works without disrupting internet connectivity, while NextDNS fails with the errors described above.

I would appreciate your help in determining why the TLS connection cannot be established and whether there are any known connectivity issues or recommended alternative endpoints for Keenetic routers.