Thanks for your ideas, Bruce. Three separate people at Cox contacted me - and rebooted Contour and the modem but were unable to fix the problem. My other streams such as Netflix and Disney+ work fine on Contour, but not HBO Max. I also uninstalled and reinstalled HBO Max, but no difference. Unfortunately I'll just have to keep switching to Firestick remote. Disappointing, but thanks again.
I am so very sorry to learn of your HBOMax app troubles and I'd like to help. If you are subscribed to our cloud DVR service, you always have the liberty to swap the box out and see if the HBOMax app works better on a different box. This way you can inform us if the issue is with the app or the box.