@dangcong 

Hi, thanks for posting question here.

EX510 is a customized model for some ISPs only. We may need to confirm further information to understand your situation better, according to your issue description"after 3-5 days, the EX510 will be offline status on the app even the internet light still green"

1.What is your exact network topology? When the EX550 shows offline on app,  can your devices still connected to EX51's wifi network? If you connect a wired device to EX510 via Ethernet cable, does the internet work?

2."The app" you mentioned is Tether app or Aginet app? Did you notice "offline" from the Easymesh part of main router page on Tether app? Did you notice EX510 showing on Aginet app?

3. Please ping EX510 referring to this guide.