Open File miniserv.conf with text editor app

nano /etc/webmin/miniserv.conf
Custom port number on line 1 (default port is 10000)

Save & exit
CTRL + x > y > Enter
Allow custom port on ubuntu firewall
home/firmanprtama# sudo ufw allow [CustomPort]/tcp
Rule added
Rule added (v6)
Example
/home/firmanprtama# sudo ufw allow 14045/tcp
Rule added
Rule added (v6)
home/firmanprtama#

Please Reboot your Server

/home/firmanprtama# reboot
Please access webmin with your favorite browser

