Its probably a PON light flashing not POS , that means the ONT is not authenticated at the headend , so although it’s been installed and spliced through and has an optical path to the headend, the ONT isn’t ‘recognised’ by the it , and if it were never authenticated, because the OR installation and authentication was unsuccessful, then the address wont be shown as ‘ready for service ‘ until it is authenticated.
Some issues on installation cannot be dealt with by the installer on the day , they pass the issue to a ‘back office’ to resolve, usually this is done before the consumer has been given the keys by the developer, if the developer was not keeping the OR new site representative advised of plots being ready for occupation then then OR can have to play catch up , not having the usual lead times etc.
You could speak to the developer, who can speak to the OR field based coordinator and find out what the delay is , it’s also possible that whatever the problem is, is already being remotely dealt with ( physically there is no OR field work to do , otherwise the LOS light would be lit ( loss of signal ) ,
If the PON light becomes steady , that signifies that the ONT and headend are now ‘communicating’ with each other , ‘ready for service’ should follow soon after that, then the address should appear on ISP portals as being available to order.
