Hello y0c_e,
Thanks for posting in the Apple Support Communities.
It sounds like you've noticed your Apple Watch is losing the cellular connection when you leave your iPhone at home, right?
If we have that correct, have you noticed if this happens while using Low Power Mode on your Apple Watch? If you're unsure, check this out as that could be related to what you're experiencing. The following resource includes more details, but we'll include details regarding Low Power Mode if needed: Use your Apple Watch without your iPhone nearby - Apple Support
If your Apple Watch is in Low Power Mode
When your Apple Watch is in Low Power Mode, several features are turned off or modified, which can affect some of the apps listed here.
• Wi-Fi and cellular data are turned off when your iPhone isn't nearby, until you open and use an app that requires a network connection.
• Incoming phone calls are sent to voicemail when your iPhone isn't nearby, and outgoing calls might take longer to make.
• Background heart rate measurements, background blood oxygen measurements, and heart rate alerts are turned off.
If that doesn't seem to be related to what's occurring, be sure you have the latest watchOS version installed to see if that may help. For assistance with updating your Apple Watch, check out this resource: Update your Apple Watch - Apple Support
Should you need additional assistance, contact Apple Support directly to check into this with you in more detail.
Have a great day!