So I want to set up a security camera remotely and I was wondering if you could connect motionEyeOS via sim instead of wifi and use it the same.
Unlikely. Your SIM would need a static IP and you would have to be allowed/able to set up a port forward to access which ever port you have motioneye running on. That sort of feature is normally only available to business customers on mobile networks and your not likely to find it as a standard customer.
The closest you could probably get would be if you get a mobile router provided by a network which would let you setup port forwarding and dynamic dns but again you'd have to find a mobile provider and a router which allows you to use those features they're not standard for mobile networks.