Thanks for your comments!
It is more clear for me...
But normally I can use the functionality NTP of my NAS...? and this case I don't need to have the second router connected on WAN port...
If you confirm, I will work harder to try to use my NAS as NTP host...
Not sure if I understand correctly what you are asking, but in case you are asking about the configuration differences between the NAS and the 2nd router running in AP mode, the difference is simple, the NAS only has a LAN (some may have two) port(s) which provides only one connection to the only network it's known to it, the Local Network, all traffic heading away from it's local network is sent to it's local gateway (which usually it's a router's IP address or a L3 switch Interface), in the other hand, the router setup in AP mode, although the configuration allows for it to be in the same network as the 1st router, (by link of their LAN ports), the LAN configuration lacks the ability to define a "Default Gateway", this due to the fact that its routing table is still well aware of its two different network interfaces (LAN and WAN) so, its default action for all traffic generated by it self for functions (like NTP) is to use it's own WAN port whenever it needs to sent traffic to the outside world.
If you by any chance have a NAS that provides a service to use as a Local NTP server, this also would only work for other network clients, but not the 2nd router configured in AP mode... for the same reasons explained above.
I hope this makes it clear... if not, fire away your questions... I'll think of a better way of explaining it.