Recover QNAP NAS

Since the day I changed my wifi router to Deco X90, the connection from my PC to NAS became unstable. Sometime i can access the NAS web page sometimes not, mostly i need to restart my NAS and wifi router and then i can connect, but sooner or later, it will not be able to connect again. Even with Qfinder pro, it is unable to locate or detect the NAS despite it is within the same network. Qfinder show "Cannot connect to device. Please check if the device and your computer are on the same subnet." I dont have this issue with my old wifi router.

Protect QNAP Data

I understand that when i change the router, all the IP address that previously set would run away and need to setup again, and i did that, even reserve a specific IP for the NAS (I did this for my old wifi router as well). But it is just weird that I could not even find the NAS in my local network.

Did I missed out something? anyone has any idea why this is happened?