Hey guys

I am really out of ideas at this point and need some help to figure out how to deal with this. 

There is a very weird network speed issue in my hand that I am trying to assist for a business network of a client. 

They have a Digital Island 1G fibre connection plugged into a Sonicwall TZ270 doing WAN via DHCP/VLAN10 (this is sort of a new connection for them with new hardware, after recent site move). There is then an Aruba POE switch carrying their local vlans and distributing network to the end devices including WiFi.

The weird issue is that for the past week every morning, before the first user arrives and in the evening around 5 PM the network comes to almost a hault. 

Normally, for the whole day there are no issues reported and my speed tests to the internet are very good (500Mbps Down/400Mbps Up). I have noticed that around 5 PM, they start seeing the slow down (immediate and not gradual) where the speed test to internet is very inconsistent. From the same PC and same speedtest server, the speeds are like this (400 Mbps Down / 1-10 Mbps up) or like this below as well.

When the issue is active, I have tried doing local speedtest using Laptop - Sonicwall - Switch - PC and also laptop - Switch - PC setup and get comfortably get speeds around 700+ Mbps, so the local network/link to firewall and switch is not impacted and its only the WAN link.

I have also used Realtime interface trafic monitoring and there's just normal traffic through interfaces and nothing is being bottlecked.

This always fixes itself once the Sonicwall is rebooted and just occurs at the next time (5 PM or before 8 AM)

I am really out of ideas and it's pushing the pressure from the users because of the Christmas period nearing. 

Can anyone suggest what can I check next or possibly look at the issue next. I haven't raised to Digital Island yet but I need to give them good proof, so the ticket is raised past the L1 team for actual checking as I have done the normal troubleshooting already.

Thank you for reading and help.