Google internet speed check. It could be your ISP, Modem and Router.

some modems are also routers and you can get wifi connectivity direct from the modem.

You can also have router to attach to the modem that will give you the wifi.

I got my speed increased when I was working from home. The cable company had to put in a new modem.

Modem: Device which establishes the connection with your ISP to provide a path to the internet. These devices are typically capable of operating at a faster speed than your actual connection (metered by your ISP) speed, but it's possible if you called your ISP and ordered a higher speed, they might not notice (or know) that your modem is not capable of that speed. Have you upgraded your speed from them since you first installed the modem?

Router: Device which allows for multiple computers to be connected to one another via wires or wireless, and typically both. They are typically capable of speeds much faster than an ISP provides for the internet (i.e. are not the limiting factor).

Most devices you receive from ISP's these days are combination modem/wireless router, but it's possible to separate them. Normally you'd do this because you want a faster network in your house (communications between your various computers directly, not involving the internet), and the networking component of your ISP-provided modem/router is behind the times (G-speed rather than AC-speed for example) ... in that case you can use just the modem part of the device (to talk to internet), and get a separate, more powerful router to handle the networking in-home.

I just got a new router, a dual band with up to 5G capability (triple band too pricey but better if you've got gamers, streamers, and surfers/developers that require a very fast and even flow to different devices simultaneously). This one hones in on the "wifi beams" and is able to handle VPN programming to multiple devices on the network (still deciding about doing that). Page loading seems much faster...so far so good with stable connectivity at a range I can use devices a fair distance outdoors!

As above, some modems do have wifi routing capabilities.

There's lots of choices and price points; ask your browser to find the 2019 best/popular product reviews.