#/lib/systemd/system/autossh.service

[Unit]
Description=Auto Reverse SSH
Requires=systemd-networkd-wait-online.service
After=systemd-networkd-wait-online.service

[Service]
Type=forking
ExecStart=/usr/bin/autossh -M 0 -f -N -R 2255:localhost:22 user@domain -g -o ServerAliveInterval=60 -o ServerAliveCountMax=3 -o ExitOnForwardFailure=yes

[Install]
WantedBy=multi-user.target

sudo systemctl status autossh.service

ssh -R 2255:localhost:22 user@domain

/usr/bin/autossh -M 0 -f -N -R 2255:localhost:22 user@domain -g -o ServerAliveInterval=60 -o ServerAliveCountMax=3