You may use the following settings in your .tmate.conf;depending on your tmate version you might have to drop tmate-server-ed25519-fingerprint:
set -g tmate-server-host tmate.tik.uni-stuttgart.de
set -g tmate-server-port 2022
set -g tmate-server-rsa-fingerprint SHA256:UkwPRZ4IP1I/Qg+c9COTIi7ZBq5XFE88jGsqEbZz3P8
set -g tmate-server-ed25519-fingerprint SHA256:P948wEFWmq39AoMzgmgE6BKxYpY3lbDg0eRjdx07aMI