We can test a internal server with private IP by serveo.

Just need one command , do not need install.
ssh -R 80:localhost:3000 serveo.net

copy and paste it to the terminal.
But, it’s not work today.

The test procedure as below.

Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.
Last login: Tue Dec 31 22:52:18 2019 from 192.168.1.200
–private ip is 192.168.1.200

root@TEST:~# ssh -R 80:localhost:3000 serveo.net
ssh: connect to host serveo.net port 22: Connection timed out
–what’s going on?
–try again, and show the system time.
root@TEST:~# date
Wed Jan 1 11:14:06 CST 2020
root@TEST:~# ssh -R 80:localhost:3000 serveo.net
ssh: connect to host serveo.net port 22: Connection timed out
–it takes 5 minutes to connect serveo.net, but fail
root@TEST:~# date
Wed Jan 1 11:18:46 CST 2020

it’s not work today, by the way, I can build a minimal linux to provide the ssh forward service that host in Google Cloud Platform(GCP).
I think this might work like serveo.