[digitallyfree]

This is the reason why I still stick with OpenVPN on TCP 443 for my selfhosted VPN. Yes performance suffers a bit but it works absolutely everywhere including behind campus/corp firewalls as no one blocks TCP 443. I've tried running a seperate UDP instance on a different port for situations where I need higher performance but for my use cases TCP works fine.

From my experience UDP 53 like another commenter suggested does not always work as some firewalls forcibly route all UDP 53 packets to their own local DNS server in order to prevent people from using their own.

As a bonus OpenVPN has the "port-share" option which allows you to share the port with other services like an SSL web server. SSLH is also an option if you want to host both your VPN and a HTTPS site on TCP 443.