Have you tried the network update? Let us know in the comments if it fixed your HDMI issue.
In 99% of cases, a fixes these issues. However, finding the correct file for the X550BT can be confusing because Denon handles updates differently than other electronics brands. Denon Avr X550bt Firmware Download- - Google
Here is the official, safe way to download and install the latest firmware for your Denon AVR-X550BT. Unlike TV or PC manufacturers, Denon does not publicly host downloadable firmware files (.bin or .zip) for the X550BT on their website. Have you tried the network update
Is your Denon AVR-X550BT acting up? Maybe the HDMI ARC stopped working, the Bluetooth is glitchy, or the unit won’t turn off properly. However, finding the correct file for the X550BT
If you see a website asking you to pay for a "Denon X550BT firmware download" or to install an "exe file," Those are scams or viruses.