To make it sort, I have successfully created MariaDB pod using YAML manifest, then exposes it as service to port 3306 using NodePort.

Until that step, everything seems normal, kubectl didn't give error messages, but MariaDB cannot be accessed from outside (external IP)

Troubleshooting steps I've done:
1) I have opened MySQL 3306 port on the firewall.
2) I enter into MariaDB environment pod/docker via the kubectl command exec MariaDB /bin/bash and then run mysql -u root -p, the database responds normally
3) I do SSH to the node where MariaDB image is located, and probing the port opened with telnet, and the results cannot be connected
4) Node's External IP that I use can be connected properly from my local PC, proofed by I can directly do SSH from my local PC.

I understand, that exposing MySQL to a public IP is a dangerous practice. I did this in order to review Kubernetes DigitalOcean service for converting an existing application to Kuberneter Digital Ocean.