Loosing connection with FG40F behind Transfer network and Vodafone router - any ideas?

Hi,

we have a small office connected with fiber from Vodafone. Their router has 192.168.1.1 and we have a transfer network to our FG WAN 192.168.1.254. With this kind of router we were not able to connect the fiber directly to the FG so we had to configure it like that.

Now we have problems for months now. Every other week for 2-3 days the users loose connection to the internet like 4-5 times a day. Later we check and everything seems normal. Vodafone sent like 4-5 times a technician to test and he connects the laptop to the router and also everything is fine, so we are doing nothing to check the situation.

Any siggestions we could do or test?

Thanks!