So you want to upgrade your DD-WRT instance on your router. You want it to work without issues and be sure there are no hiccups along the way. Then I recommend that you connect to the router via SSH and execute the following commands for the best experience.
cd /tmp wget http://download1.dd-wrt.com/dd-wrtv2/downloads/betas/2020/11-03-2020-r44715/netgear-r9000/r9000-webupgrade.bin write r9000-webupgrade.bin linux && reboot
You can navigate to the download location through the DD-WRT website to get and copy the link to the download (replace the https with http for it to work on the router).