I have used this trick when I in high school grade.

Just setup openvpn with port 53 UDP with cheap vps, then connect it, and get unlimited internet access

Internet connection in my country is very limited and expensive in that day

You have no idea how badly I was looking for a way to tunnel via airplane wifi. Even going as far as trying to use the "free tier" fb messenger access to move packets. Now the ToS have a very low bandwidth limit on the free tier (eg. 100mb), so I'm not sure if it'd work for any real application.

I never thought to use DNS as a tunnel, but I have seen some pretty interesting loopholes that I was trying to find. Some airline's captive portal used to rely on some google service (captcha probs), and for a period of time didn't block any google-owned IP address that wasn't a consumer website (eg search, yt) so GCP and developer docs would load if you had the URL. I couldn't reliably get to user-generated content through it though.

> I never thought to use DNS as a tunnel

Most firewalls drop or hijack packets on port 53.

I have experienced this, i.e., hijacking, but I would respectfullly contest the term "most". But perhaps the statement was only referring to airports. I could be wrong but I like to think it is a minority of networks that hijack or block DNS. For example, a minority of consumer ISPs, other networks controlled by third parties (airports, hotels, etc.) or countries that conduct extremely aggressive internet censorship.