“Buy the modem or keep renting it?”

Your son is right on both counts. Buying usually saves money, and plenty of modems won't work with a given provider. The trick is to do it in the right order: figure out what kind of service you have, check the approved list, then do the math.

First: what kind of internet is it?

The answer changes depending on how the internet arrives at your house.

• Cable. Most cable providers let you use your own modem if it's on their approved list. This is where buying usually makes sense.
• Fiber. There's often a provider-owned box called an optical network terminal that stays theirs. You may be able to use your own router behind it, but there's usually no modem to buy.
• DSL. Some providers allow customer-owned modems; check.
• Fixed wireless or cellular home internet. The equipment usually comes with the service and isn't replaceable.

Also check whether your home phone runs through the same box. Voice service over cable often requires provider equipment, and buying your own modem can mean losing the phone line or needing a separate adapter.

Second: what you're renting

Some providers rent a single combined box called a gateway, which is a modem and a Wi-Fi router in one. Others rent the two separately. Your bill line should say which. If you replace a gateway, you need both functions: either your own combined unit, or a modem plus a router.

Your provider's broadband label lists equipment fees for your plan, and the fees section of your bill shows what you actually pay.

Third: the break-even math

Here's the arithmetic. These numbers are an example to show the method, not prices you should expect.

Line	Example	Yours
Monthly equipment rental	$15
Cost of a compatible modem	$120
Cost of a router, if you need one	$80
Total to buy	$200
Months to break even (total ÷ monthly rent)	about 13
Savings over three years (36 × rent − total)	$340

If you plan to stay with the provider and the break-even is a year or two, buying usually wins. If you're likely to move or switch to fiber soon, the case gets weaker.

Fourth: buying the right one

1. Go to your provider's website and search for its list of approved or compatible customer-owned modems.
2. Match your speed tier. A modem that's approved but rated below your plan's speed will hold you back. The list often says which models support which tiers.
3. Match the standard. For cable, the list will name a DOCSIS version. Buy what's listed, not just what's newest.
4. Buy from somewhere with an easy return policy, in case activation goes badly.
5. Keep the box until it's activated and working for a week.

Fifth: switching over

You'll usually activate the new modem through the provider's app or website, or by calling. Have the modem's identifying number from its label ready.

Then return the rented equipment. This step matters more than it sounds. Providers can charge for equipment that isn't returned, and disputes about whether a box arrived are hard to win without proof. Return it the way the provider says to, get a receipt with a tracking or store reference, and check the next two bills to make sure the rental line is gone.

When renting is fine

• You need the provider's phone service and there's no workaround.
• You expect to move or switch provider within a year.
• You want the provider to handle replacement if the equipment fails, and that's worth the monthly cost to you.

Those are real reasons. "We never thought about it" isn't one, and that's most of us.

Your son's job

Hand him the approved list and the bill. Ask him for the model, the price and the break-even month. If the break-even is inside two years and you're not moving, buy it.