When network traffic passes through firewall my ISP's router stops working.

Hi!

I recently bought a USG60 firewall. It's mostly has its default settings. I have only configured an SSL VPN and enabled the UTM services.

The WAN interface has a public IP address and gateway address provided by my ISP. The issue I am having is that when the devices on my local network have my firewall as their gateway, my internet partially stops working 2-3 times a day. I say partially because most websites work, but things like WhatsApp calls, speedtests, remote connections over VNC stop working.

The only way to resolve the issue is my resting my ISP router. I have contacted my ISP but they insist it's not an issue on their equipment and they actually changed it once because I requested it.

The thing is that when I stop sending traffic through the firewall (change my networks devices' gateway to my ISP router IP address) then this issue does not occur. 

Any ideas on what could be happening?