1. Do you have a public_html folder?
  2. Make sure that your public_html folder contains a .htaccess file with the right contents
  3. Fix permissions on public_html
  4. Bonus:

Sometimes you may not be able to run PHP files or view HTML in a web server. This can be related to permissions and/or the lack of a .htaccess file in your public_html folder.

Do you have a public_html folder?

ssh upXXXXXXXX@gnomo.fe.up.pt
ls -la 

Do you see a public_html folder? If it does not exist, run the following command:

mkdir public_html

Make sure that your public_html folder contains a .htaccess file with the right contents

cd public_html
vim .htaccess

paste the following contents:

Options +Indexes

To save and exit, press :wq

Fix permissions on public_html

cd ..
chmod -R 0755 public_html

Bonus:

Guide on how to configure the VPN in FEUP.