OK, problem resolved, this is what DTV Tech Support says what happened: On January 1st the AT&T merged billing systems (for landline phone and DirecTV) dropped (behind the scenes) a "4K Service" $0.00 line which does not appear on our bills and can only be seen by their reps. Once the rep added the "4K Service" back to my acct. the 4K channels opened up again and error code 721 disappeared. So trying to refresh signal (online or on the phone) to get code 721 to go away never worked because it was being overridden by the missing "4K Service" on the account. As a routine matter, the "4K service" is automatically added to all accounts activating 4K equipment but on January 1st it got deleted. Hopefully it won't be an annual thing. If anyone has this problem just know to call tech support and ask for the "4K service" to be added back to your account on their end. There's nothing you can do on your end to fix it without calling them.