You said you tried to update the Thunderbolt driver, but based on the error message and version number, it sounds like you tried to update the firmware. in order to update Thunderbolt firmware, you have to have both the latest system BIOS and the latest Thunderbolt driver/software installed first.
I have the same exact problem, after Thunderbolt FW Upgrade HDMI gone. Tried with two different monitors.
Also you can't downgrade the Thunderbolt FW
We have now removed the Thunderbolt firmware 4.26.03.001 ,A07 from the Support site and replaced it with A06 (link) until we can validate the A07 firmware works correctly.
In the meantime, if you have installed the A07 firmware then if you can roll back to the A06 version using the method below and it should resolve the issues that you all experienced:
Dell | Social Outreach Services
I'm based in the UK and available Monday to Friday 9am-5pm GMT
I could roll back the firmware from NVM26 to NVM21 on my XPS 9560 and the HDMI port is working again.
Thank you very much!