I have couple of RuuviTags and successfully updated all but one. The one remaining is not entering the DFU mode with the button sequence. Apparently, it just switches between the RAW and URL mode. It has probably some quite old firmware. Reboot button then broken maybe?
Reboot button must be broken since on other device the red LED blinks on reboot but this is doing nothing. However, then I thought that maybe RuuviTag enters DFU actually when B is pressed while the device reboots. So then I tried pressing B and disconnecting the battery briefly. That took me to DFU and I could install the 3.28.12 firmware to get the latest features for iOS! Nice! Problem solved.