Installing dependency not satisfiable no libcurl4

I want to install the new version of vpn-unlimited. When I tried to to install the supplied .deb file I get a dependency error.

I am using Stretch (debian edition) of Mint.

Instructions found in Google
https://dev.to/jake/using-libcurl3-a...04-bionic-184g

Suggested I copy libcurl version 4.4.0 extracted from libcurl3 as lib curl.so.3 (instead of 4). Tried this still cannot install vpnunlimited error could not fin libcurl4 dependency.
Then
Copied the libcurl.so.4.4.0 as libcurl.so.4 in /usr/lib still does cannot install vpnunlimited installer.

ALso
$ whereis libcurl.so.3
libcurl.so: /usr/lib/libcurl.so.3

But if I type
$ whereis libcurl.so.4
libcurl.so: /usr/lib/libcurl.so.3

Any insight into this will be greatly appreciated

Best regards