http://security.ubuntu.com/ubuntu/po...4.15_amd64.deb Worked for me.
If you still can't compile the modified rtmpdump and librtmp.so.1 you can create a live usb of linux 18.04 and compile it there. After that make a backup of /usr/bin/rtmpdump and /usr/lib/x86_64-linux-gnu/librtmp.so.1 and replace the ones on your ubuntu 20.04. After that just dpkg install the libssl from above and it should work without returning any errors (hopefully). It worked for me.