Update iPad mini 1st gen

Yup,

The 1st gen iPad Mini cannot upgrade/update beyond iOS 9.3.5/9.3.6 for cellular data models.

Your iPad Mini is nearly 8-years old now!

These old 8 and 9-year old iPad models are too old, now, too underpowered and completely incompatible with any of the current and future versions of iOS/iPadOS.

Also, tap and read my link below for my additional info about your nearly 8-year old iPad model.

If you own OLD 8 and 9 Year Old iPad mo… - Apple Community

Sorry & Good Luck to You!

What's the current version number on the device in Settings > General > Software Update? The last iOS version released for the iPad mini (1st generation) is iOS 9.3.5 (or 9.3.6 if it's a model with cellular), so if you have this, the device is already up to date. Well, it's outdated software, but it's the newest available for this particular device.