I'm experiencing similar difficulty trying to upgrade my Shelly 2.5 from v1.11.7 to v1.12.0 - the upgrade process appears to hang. Looking at a packet capture, the firmware download never begins except to retrieve metadata. The only indication that anything is meant to be happening is from http://shelly-ip/ota which shows status: "updating".
I've tried
- factory reset
- downloading the firmware .zip & updating the Shelly directly from a server on the same network
- upgrading to v1.11.8 via http://archive.shelly-tools.de/
The outcome seems to be the same every time.
My other Shelly 2.5 which shipped with v1.11.8 was able to upgrade to v1.12.0 with no problem.
For now v1.11.7 mostly works, but randomly (every ~6 hours on average) the device becomes "unavailable" to Home Assistant for about 45 seconds. I saw the same with the other Shelly 2.5 running v1.11.8 - I am trying to update them both to see if this issue has been mitigated 
EDIT: Tried again today & it updated instantly. No idea what changed, at least it's resolved fro now.