Hi and welcome to JustAnswer,Thank you for the question and your patience. Should you still need assistance for information purposes only...Initially, please bear with us; re-confirm and post back with the complete model number of the "...8 year old Asus laptop..."
Additionally, does the description of "...HDMI Cable does not work..." mean that the laptop has an HDMI port, can be connected to the PH300 but no video?
Appreciate the additional information.
Have you ensured that the HDMI video out is enabled in the A6? ...and checked / set the screen resolution of the Asus A6-11C ?
Apologies, Internet is sluggish if at all.
Please set the screen resolution to 1920 X 1080 or lower @ 30p