Reading those last postings I realize that I have wrongly understand that you have to set IP's to static because that is required by your mobile hotspot, which lead me to wrong track.
Sorry for that miss understanding.
In some users (like myself) keeping router on pure DHCP and disabling IPv6 has been helped, but I see form previous postings that you have already try that(?).
There should be some kind of table on your router (DHCP) settings like I have on my old router. By clicking button "Manual Assignment" that router remember which IP-address belong to each MAC address. (Even while this IPv6 trick work for me didn't put my router back to full automatic and that's just laziness.)
New routers / modems may remember this automatic and no changed required.
Have you look your router manual for IP-assignment?
Which is branch and model of your router?