Hi @SobyKuruvilla I'm not sure if you can change one of those ports to a WAN port. I'll let another comment there.
However, for a Port-channel to work the media types need to be the same. You can't bundle a fibre port with a copper port.
Darren OConnor | doconnor@resalire.co.uk
https://www.linkedin.com/in/darrenoconnor/
I'm not an employee of Cisco/Meraki. My posts are based on Meraki best practice and what has worked for me in the field.