If there's no ForceCommand (I think that's what it was called?) in sshd.conf and no command=x in .authorized_keys (if any), you can probably use mosh. It spins up SSH to authenticate and then immediately start a UDP based connection afterwards. Instead of logging out every 15 minutes, you will get a "Last contacted X seconds ago" overlayed every minute, that's sure an improvement. Do note, however, that it keeps your shell running with no way to reattach to it if your machine turns off.