All connected devices (Wifi and via Ethernet) lose internet access intermittently and simultaneously

We've been having a nightmare with our internet accessibility over our Vodafone VDSL connection - it's been going on for almost two months now, and Chorus, Vodafone, Bose and D-Link have been involved in trouble shooting (unsuccessfully).

The issue we're having is intermittent, but simultaneous, stoppages across all our connected devices in terms of functional internet access - the modem says there's a connection, but none of the connected devices seem to have access. When the problem occurs, devices connected to the modem via Wifi and Ethernet are affected (i.e. it's not just a Wifi issue). Our setup includes:

Vodafone VDSL connection with 17Mbps down / 1.5Mbps up Vodafone Ultra Hub modemOne smart TV connected via EthenetOne Bose sound system connected via EthernetFour/Five mobile phones (Wifi)Couple of iPads (Wifi) Four laptops (usually Wifi)

We did have a D-Link Mesh Wifi system connected via Ethernet (with all non-Ethernet connected devices using the Mesh Wifi, not the Ultra Hub's Wifi), but have got rid of the D-Link Mesh system following advice from Vodafone that it may have been the cause of our issue. Vodafone has, on three seperate occasions, run 24-Hour line tests using their Chorus portal - no issues detected.

When the second and third 24-Hour line tests were being performed, I ran a continuous sequence of Ping tests from my MacBook while connected to the modem via Ethernet cable. Notwithstanding that the line tests showed no issues, my Ping test results showed multitudes of slow and timed-out Pings. Vodafone have suggested that we are 'maxing out' the modem - I don't know what to make of this. I doubt we're high-demand in the scheme of things (one HD Smart TV, no gamers, bit of music and kids on social media), so surely a fit-for-purpose modem should handle that - if the line speed isn't up to servicing the demand from the connected devices, I'd expect the modem to ration out traffic rather than freak out and serve up nothing?

Any help would be fantastic - I'm happy to share the result of my Ping tests if that may be informative. Thanks, Simon