@BPE_llc302-112a,
They would be correct; they just did a not so great of explaining what is actually happening and why it has to happen.
Firstly, what you are referring to as a Modem+Router device is officially coined as a Gateway
To create a stable network that allows your router to send data to the correct devices, your network has to run a single DHCP server for all connected devices. DHCP servers are the yellow pages for your network, and allows your router and devices to differentiate from each other as it assigns a local IP address (192.168.XX.X) to each device. In traditional setups, these servers are run by the router on the network.
If there is more than one DHCP server running, data will get lost, and devices will have trouble correctly identifying devices and connecting to the network. For this reason, as you allude to, you cannot have more than one router on a network providing a DHCP server, at least without very advanced configurations.
As they mention, "PassThrough Mode" or "Bridge Mode" will be a setting on your Gateway that will disable the router portion of the device, including the wireless network and the DHCP server, while still operating as a conventional modem. Your router should be able to automatically communicate the static IP address with the modem as normal, or it can be set manually.
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The other option is to set up your router in AP mode and leave your Gateway as is, your Archer will then essentially serve as a wireless extender for the network provided by the Gateway. The caveat to this is that it will disable many of the advanced protection, routing, and management options of the Archer, as all of these will be managed by the Gateway's Rotuer and DHCP server