Firstly I'm with Bigpipe on 100Mbps Fibre. Fibre was installed under Vodafone last year and we had no problems with the connection (we only left them because they kept stuffing up our bill).

So for the last few weeks the internet for all devices connected on wired and wireless networks get disconnected from the internet (still connected to the modem/router though) exactly every 4 hours for exactly 10 seconds before every is reconnected. We have not changed anything on our network (apart from maybe a couple of new devices connected), modem/router settings and cabling is all the same as it was on Vodafone. If I am loading a page in a web browser like Firefox it shows a Network Protocol error:

And Netflix and TVNZ On Demand show errors at the same time. I know the timing of disconnection exact because it shows up in the router logs as these events (IPs left in because it has changed since then):

[Time synchronized with NTP server] Monday, Feb 18,2019 11:40:10
[Internet connected] IP address: 122.58.118.229, Monday, Feb 18,2019 11:40:09
[Internet disconnected] Monday, Feb 18,2019 11:39:59
---
[Internet connected] IP address: 122.58.118.229, Monday, Feb 18,2019 07:40:02
[Internet disconnected] Monday, Feb 18,2019 07:39:52
---
[Time synchronized with NTP server] Monday, Feb 18,2019 03:39:55
[Internet connected] IP address: 122.58.118.229, Monday, Feb 18,2019 03:39:55
[Internet disconnected] Monday, Feb 18,2019 03:39:45
---
[Time synchronized with NTP server] Sunday, Feb 17,2019 23:39:48
[Internet connected] IP address: 122.58.118.229, Sunday, Feb 17,2019 23:39:48
[Internet disconnected] Sunday, Feb 17,2019 23:39:37
---
[Time synchronized with NTP server] Sunday, Feb 17,2019 19:39:41
[Internet connected] IP address: 125.239.43.126, Sunday, Feb 17,2019 19:39:41
[Internet disconnected] Sunday, Feb 17,2019 19:39:30
---
[Time synchronized with NTP server] Sunday, Feb 17,2019 15:39:34
[Internet connected] IP address: 125.239.43.126, Sunday, Feb 17,2019 15:39:34
[Internet disconnected] Sunday, Feb 17,2019 15:39:23
--
[Time synchronized with NTP server] Sunday, Feb 17,2019 11:40:32
[Self2WAN ICMP type b Detected!] To prevent from revealing router's activity, this packet is droppe Sunday, Feb 17,2019 11:39:28
[Internet connected] IP address: 222.153.159.198, Sunday, Feb 17,2019 11:39:27
[Internet disconnected] Sunday, Feb 17,2019 11:39:16
---
[Time synchronized with NTP server] Sunday, Feb 17,2019 07:40:01
[Internet connected] IP address: 222.153.155.6, Sunday, Feb 17,2019 07:39:19
[Internet disconnected] Sunday, Feb 17,2019 07:39:09
---
[Time synchronized with NTP server] Sunday, Feb 17,2019 03:39:13
[Internet connected] IP address: 122.58.135.119, Sunday, Feb 17,2019 03:39:12
[Internet disconnected] Sunday, Feb 17,2019 03:39:02
---
[Time synchronized with NTP server] Saturday, Feb 16,2019 23:39:11
[Internet connected] IP address: 122.58.135.119, Saturday, Feb 16,2019 23:39:05
[Internet disconnected] Saturday, Feb 16,2019 23:38:55
---
[Self2WAN ICMP type b Detected!] To prevent from revealing router's activity, this packet is drop Saturday, Feb 16,2019 19:39:01
[Internet connected] IP address: 125.239.40.89, Saturday, Feb 16,2019 19:38:58
[Internet disconnected] Saturday, Feb 16,2019 19:38:48

I have contacted Bigpipe and they told me:
"the diagnostic on the Chorus ONT shows a historical timeline which has had zero optical interruptions. A number of things could cause this error, such as the modem losing connectivity to the ONT, or the ONT losing connection to the Chorus network, but this would reflect on our historical line test, and it hasn't. I cannot book a technician for a fibre connection where optical levels are perfect as Chorus will reject the fault. One more thing you can try is replacing the ethernet cable between the ONT and the modem."

I don't think it would be the ethernet cable from the ONT as it is a good quality Cat6 cable I only bought last year. I have tried rebooting the modem and updating its firmware to the latest version (as it was on quite an old version) but the dropouts still happen every 4 hours.

Because of the exact timing of the dropouts and the changing of public IP most disconnects, is this some kind of IP lease time issue?

[Mod Edit: Murph - Moved to BigPipe subforum]