@ivanmanzanolh
I tried to set a password directly from phpMyAdmin with MySQL. When I try to log in, it says "Invalid credentials".
No, its not possible password setup with directly from phpmyadmin. First you will login with any admin and using the same password string in another agent or customer so that login with the same password.
If you added directly from phpmyadmin so here shown direct password instead of password string like above customer email:

Then this is not a login with this password, also showing a invalid credentials.
So you will setup of the swiftmailer, and email settings. After setup you will received a create password email.
For swiftmailer setup you can follow the below blog:
https://www.uvdesk.com/en/blog/setup-gmail-mailbox-uvdesk-open-source-helpdesk/