How much effort have you actually put into trouble shooting the issue? Maybe it is just a wrongly set CPU governor (performance, instead of ondemand or something else)? Or a certain kernel flag that have to be set on boot?
Modern Android versions can use DoH (DNS over HTTPS) which can not be intercepted. If you don't have this option or are not sure how to configure it, you could use the Quad9 app to enable secure DNS.
This way you can make sure it is not related to DNS. Frankly, I can't imagine they are blocking the IPs of the DuckDNS servers directly.
I don't think non-LTS Ubuntu releases are technically rolling releases. They are more like shorter supported snapshots.