Router DCHP fails when connected to highspeed internet

Last Tuesday I upgraded my internet from Spectrum Communications from their basic package to a residential package that gets between 100Mbs and 200Mbs. The basic package maxes out at about 37Mbs (NetFlix has some minor issues). The router worked just fine for 2 years prior to upgrading the internet service.

For the first 16 hours everything was fine, then my Zyxel USG 60 router stopped working. This is a problem because I have 3 working computers and 3 entertainment devices, a Samsung smart TV, a Sony ZA3100ES surround sound stereo and a Sony BluRay DVD player, 3 or 4 of which need to be connected at the same time. I can currently connect 1 and only 1 device to the modem at a time. When I try to connect a second computer I get errors and when I diagnose the problem I get Invalid IP Configuration.

I have a second router which I tried, a Zyxel USG 40. There is no difference when using the second router.

None of the routers are supplied by the ISP.

I know it is the router because I can connect a single device to the modem and it works, when I add the router back in it fails. I suspect that the Modem DHCP server is the issue, previously all the IP addresses were being assigned by the router until I upgraded the internet service. If I disconnect the modem the router works fine for controlling the LAN using either DHCP or Static IP Addresses, but when I connect the modem back to the router everything fails again.

I’ve spoken to Spectrum Communications Tech Support twice, both times they said call my router company.

Questions

1. How do I disable the Modem DHCP?
2. Is the modem somehow putting my router into bridge mode? If so, how do I get my NAT back up and running?
3. What setting do I need to change on my router to make it work?

Modem

• Hitron E31N2V1
• Currently fully controlled by Spectrum Communications

Router 1

• Zyxel USG 60
• No wireless, everything is connected by Ethernet, either CAT 5E or CAT 6.
• Default settings except for the password
• Latest firmware is installed.

Router 2

• Zyxel USG 40
• No wireless, everything is connected by Ethernet, either CAT 5E or CAT 6.
• Default settings except for the password
• Out of date firmware.

Switches

• NetGear 5 port unmanaged switch
• NetGear 8 port unmanaged switch
• The Sony ZA3100ES also has an 8-port switch

Computers

• Dell Precision 7740 Mobile Workstation
• Lenovo P51 Thinkpad
• Raspberry Pi 4

The Ethernet cable from the modem to the router was from the Modem Ethernet RJ45 connection to WAN 1 (p1) RJ45 connection on the Zyxel USG60 Router.

One Ethernet cable from the p3 (LAN1) RJ45 connector to the NetGear 5 port switch (not pictured).

I have a new router on order because the Zyxel USG 60 and the Zyxel USG 40 are End of Life.

Solution 1:

You can connect to the router (best bet is to use a cable) and rummage around in it's configuration. Most of those can be reconfigured by the user, download the relevant manual and study it. Some ISPs (like mine here) gives you a box with very limited end-user configurability. Your mileage may vary.

The connection of the router to the wider Internet shouldn't make a difference, unless your ISP insists on taking control of the router (and not finding "their" router just disables it or misconfigures it somehow).

A broken/messed up router (specially as it worked for a while before breaking down) sounds more likely. Check if it can be updated, carefully do so, perhaps wipe and reconfigure from scratch.