Holy cow. There’s no way that’s the best solution!
I’d guess there’s still remaining bugs in Webmin’s support for the new Ubuntu network config system (it may be fixed in the next version, which I’m rolling out today). Just configure the network outside of Webmin.
It’s insane to install 16.04 and upgrade to 20.04. That is not a good solution. If you want to use the old network system (which is what this gets you), just use the old network config system. You don’t have to start on 16.04 to do that.