Hi amollokhande,
Thanks for coming to Apple Support Communities. We'd love to point you in the right direction so that you're able to continue using your iPhone Personal Hotspot without issue.
As Ubuntu isn't directly supported via USB tethering with Personal Hotspot, we may recommend reaching out to Ubuntu support resources for assistance via the following link:
Support and management | Ubuntu
We'd also like to share a great resource that has troubleshooting steps that may help:
If Personal Hotspot is not working on your iPhone or iPad (Wi-Fi + Cellular)
Have a great day!