Cannot connect anymore to 2.4 Gh Network AC87U

From this morning I cannot connect anymore to the 2.4 networks on the router. The 5Gh works fine. All was working "fine" with reboot every few days - for the last 6 months.

Things I tried:

1. Restore last backup using NVram V 21.
   
2. Reset to factory settings - the long way using the WPS button while turning on the router.
3. The router was working with Marlin 378.53 - I upgraded to the latest 378.56 beta 1....

Cannot connect anymore to 2.4 Gh Network AC87U