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.
yes, like rohtua said they are not standard but if you pay for mobile data on the sim card and ngrok premium you can set up an ngrok port forward
apt install ngrok && ngrok auth <your token> && ngrok http <your motioneyeos port> and access it n a web browser like this
<name you chose in ngrok panel>.ngrok.com:80 i am pretty sure
there is a free version of ngrok but first of all it stops after 3 hours and also it gives you a random url every time so a start script wont be helpful, ngrok is great and useful for other projects too and its not that expensive really