First, you need to buy an ethernet adapter to connect your phone to the ethernet cable. Make sure the ethernet adapter you buy is compatible with the type of phone. Many manufacturers of ethernet adapters will list which devices their products are compatible with. Also make sure the adapter matches the type of USB port on the phone. Currently, most phone ports are either USB-C or micro-USB.
If you want to connect two devices (e.g. a phone and a laptop) to one ethernet cable: Run an ethernet cable to a room, and connect it to a device called an ethernet switch. Some people run ethernet cable above or below the living space to get it out of the way, but that's not always feasible. You need two more short ethernet cables to connect both the phone and the laptop to the switch. And the laptop will need its own adapter if it doesn't have an ethernet port.
And, of course, both the phone and the laptop need to be in airplane mode for you to get the full benefit of turning off wireless. And hopefully then you can then shut off the wireless from the modem / router. Ideally, electrically sensitive individuals will want a modem that does not have the capability for wireless. On wireless modems, you can often turn off wireless through the software. However, many people have noted that the wireless sometimes it comes back on unexpectedly--perhaps because of software updates or if the modem resets.
One selling point may be that you will use less phone data (but it would only save you $ if you do not have an unlimited plan). Also, ethernet is often faster and more stable than a wireless connection.
Below is the one that I like and do well with. I am chemically sensitive and I bought quite a few that I did not do well with because I am chemically sensitive (to solvents and other chemicals).
SAMSUNG Multiport Adapter - USB C, USB C, HDMI FHD& RJ45 Silver
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