Cannot access modem page directly, only through router

I have 3 modems. Two are dying, so I bought a third one of a different brand. On the old modems, I could tell if they were working or not by accessing the config page. When they stopped working, if I plugged into them directly (or through the router) and tried to access that page they wouldn’t show up.

On the new modem, I can’t access the page unless I’m attempting through my router. What the heck is going on? Any Google search on the topic gives the reverse situation.

• The pages all have the same address, 192.168.100.1 .

• The two dead/dying ones are model Motorola MB7621.

• The new one is a Netgear CM700.

• They are all modem-only units.

• My router is a Google Wifi puck. I can still access my local devices when the internet is out.