Welcome to the forum and thank you for posting.
After checking the status of our IP-Gateway, I notice you have twenty plus CPE (customer premise equipment) on the ARP table.
Has your network always have this many devices?
Additionally when did issue initially started and did you add any addition devices?
In fact we used to have a lot more devices connected, as we are a computer training company. We have not installed any new or more devices, in fact we have reduced the number of PC's connected. Also not that most of these connected devices are rarely actually connected to the internet. On average I'd say we only have 2-3 devices actively connected to the internet at any given time.
The problem started manifesting itself some time next week.
It is very interesting to note that you reduce the number of CPE and connectivity issue continues (or worsen).
I recommend this to start on path to resolution:
If possible over the weekend (or at least 24hrs) connect a single cpe directly to IP-Gateway do a continuous ping test.
This initial is meant to check the status of IPGW.