How do I troubleshoot personal hotspot mode failing to issue IP addresses?
This one drove me nuts but I believe this will work for most of you:

Basically the APN settings for the Hotspot were missing.

If you can browse the internet on your phone and you can activate your Hotspot but you get the yellow exclamation mark on the computer's wifi or the 'Network cable unplugged" with USB tethering, try the following:


Open Settings -> Mobile Data -> Mobile Data Network
Check the APN/Username/Password settings under "Mobile Data" (most
of the time username and password - if used - are identical, if not search APN settings on your provider's homepage)
Enter the same settings under "Personal Hotspot" further down on the same settings page
Return to Settings Menu
Switch Mobile Hotspot off and on
Connect to your Hotspot -> Internet on your computer should be working now


The problem appeared for me while roaming/after changing sims, basically every time the network settings get reconfigured and the APN settings automatically change for the roaming network/new network. 

Hope it works for you too!