Recently I updated my R7800 router from 19.01 to 21.02 an the update went very well!
I was worried that the necessary package for DSL access (dsl-lite) wouldn't be installed during the upgrade, but it was! So internet was accessible during every step of the process.
Second positive result: I had problems with ds-lite in the 19.01 release and had to restart my network every night with an cron job. In the new release ds-lite was also upgraded to version 8.0 and now everything is working like a charm.
Thank you to the developers for their excellent work!