@Bryle 

Not sure if it is DHCP issue. I feel like it could be more related to DNS server.

Do you have stable Intenret on a static IP PC?

You may run a ping test on windows PC. Command ping 192.168.0.1 -t

This test will show you if it is a local network issue, or it is actually related to WAN/DNS

  @Somnus I can assure you that ISP is fine, rebooting the router solves the issue. Can also confirm that static IP also does not work. DHCP Lease is 24 hours. Once DHCP lease is done it won't renew and we will be needing to reboot the router for it to work again.

  @Bryle 

But if it is DHCP issue, it should not influence static IP device's...Static IP means the device won't require DHCP server. ..

I'm using ER605 and I never see DHCP release time make disconnection on my network. Lease time only set up to 120 min.

It will be helpful if you can find some evidence from the log. I think you can use Omada Controller to manage the router, so the Log will be saved on Controller PC.

  @Somnus I'm not sure if this is a DHCP issue or what. Internet connection dropped last saturday midnight, then rebooted in the morning, connection dropped again at midnight. I'm 100% sure that our ISP is fine. I've seen DHCP issues in here that's why I suspected that DHCP is the problem.

Also saw this one: https://community.tp-link.com/en/business/forum/topic/584136