Hotspot between iPhone and ipad
How to keep my iPhone hotspot connected to my iPad at all the time
iPad Pro, iPadOS 16
How to keep my iPhone hotspot connected to my iPad at all the time
iPad Pro, iPadOS 16
It remains connected as long as you do not turn off any of the devices.
Please see the pic below to make it clearer that Hotspot is enabled by a Carrier on one SIM and not by the other Carrier in the same iPhone (Dual SIM).

Use these steps first
Restart the iPhone or iPad that provides Personal Hotspot and the other device that needs to connect to Personal Hotspot. Make sure that you have the latest version of iOS. On the iPhone or iPad that provides Personal Hotspot, go to Settings > General > Reset, then tap Reset Network Settings.
If Personal Hotspot is not working on your iPhone or iPad (Wi-Fi

Also, check the following
Ask the Carrier to change the SIM if nothing works.
Hotspot between iPhone and ipad