I have the exact issue with a fresh Mint 20.3. After renaming the source to be focal I get farther, but still have:
The following packages have unmet dependencies:
zerotier-one : Depends: libssl1.0.0 (>= 1.0.2~beta3) but it is not installable
E: Unable to correct problems, you have held broken packages
the version installed of libssl is:
$ apt show libssl1.1
Package: libssl1.1
Version: 1.1.1f-1ubuntu2.12
Priority: important
Section: libs
Source: openssl
Origin: Ubuntu