Hi @Ash24
Looks like others have already posted answers. Unfortunately, the functionality of the port cannot be changed (i.e. from LAN to WAN). Hence, you would need to purchase SFP's for WAN uplinks on MX250.
For your reference, I have attached a KB here: Small-Form Factor Pluggable (SFP) and Stacking Accessories
If you found this post helpful, please give it Kudos. If my answer solves your problem, please click Accept as Solution so others can benefit from it.