My iPhone is no longer appearing in Wi-Fi menu above the line after changing mobile phone provider
The Instant Hotspot is part of Apple's Continuity and Handoff features.
Try each of these troubleshooting steps:

Confirm both devices are logged into iCloud.
Toggle WiFi and Bluetooth off and on for both devices (for iOS, use the Settings app and not Control Center to toggle these settings).
Reboot both devices.
Log out of iCloud on both devices and log back in.

Logging out of iCloud and back in on both devices can be a pain and take some time, so make sure to try the previous steps first. It may or may not be helpful to reboot your devices after both logging out and logging back in.
For this particular situation, the asker indicated that they did the following to solve their problem:


Logged out of iCloud both on Mac and iPhone.

Rebooted both devices.

Logged into iCloud on both devices.

Rebooted both devices.