I am not sure what you meant by Skymobile does not support cellular. I am assuming that you meant that it doesn’t support cellular service for the Apple Watch.
The Apple Watch Ultra 2 has Wi-Fi and Cellular (+GPS) capability. It will work just fine with Wi-Fi and / or a Bluetooth connection with your iPhone SE 2020. The only thing is that when you are away from your iPhone and away from a Wi-Fi network, then you will stop getting notifications until it gets within the iPhone’s and/or Wi-Fi network’s range.
In my opinion, you will be fine with a Wi-Fi+GPS only Apple Watch (Without the Cellular option). This can be a Series 9 or a Series 8 Wi-Fi+GPS only Apple Watch. I state that because you are not going to get cellular service for at least a year or more for the Apple Watch due to your contract with Skymobile. You will even save some money in the process. Look into it. Decision is yours.
Axel F.