I am still unable to get get the public key using the command,
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys B5B116B72D0F61F0
googling the error I get, reveals a likely issue with the dirmngr DNS resolver, I still can’t understand why this is happening. I tried enabling the --standard-resolver in ~/,gnupg/dirmngr.conf but that didn’t make any difference.
However, I managed to import it manually using the following instructions, and the key from this page.
I am now able to update timeshift from the official repo.