First, test properly. Plug a laptop directly into your router with an ethernet cable and run a speed test at fast.com. If that number matches what you're paying for, your ISP is fine — the problem is your Wi-Fi. If it doesn't, now you have a real case to call support with. Do this before anything else.
Now the fun part: your Wi-Fi. The number one issue in 90% of houses is router placement. The router should be roughly central to where you use devices, up off the floor, out in the open. Not in a closet. Not behind the TV. Not in the basement corner where the cable comes in. Radio waves don't like concrete, metal, or water (and that includes aquariums and human bodies).
Move to 5GHz for anything that matters. Modern dual-band routers broadcast both 2.4GHz and 5GHz networks — 5GHz is faster and less congested but has shorter range. In most homes you want to forget about 2.4GHz for everything except smart bulbs and Echo devices that only support it. On your phone and laptop, connect to the 5GHz SSID specifically.
Check your channel. If you're in an apartment or dense neighborhood, your neighbors' Wi-Fi may be stomping all over yours. Download a Wi-Fi analyzer app (NetSpot on Mac, Wi-Fi Analyzer on Android) and look at which channels are least crowded. Then in your router's admin page, set the 2.4GHz network to channel 1, 6, or 11 (whichever is cleanest) and let 5GHz auto-select.
If your router is more than five years old, replace it. The difference between a 2018 AC1750 and a 2024 Wi-Fi 6 router with MU-MIMO is enormous, especially when multiple devices are active. A UniFi Dream Router or a TP-Link Archer AX55 runs about $200 and will outperform anything your ISP rents you for $15 a month.
For larger houses, a mesh system beats a single router with extenders every time. Extenders repeat the signal on the same band, which cuts throughput in half. Mesh nodes have dedicated backhaul. I use Ubiquiti, but Eero and TP-Link Deco are excellent consumer options. Run ethernet to each node if you can — wired backhaul is night-and-day better than wireless.
Finally, the nuclear option that nobody wants to hear: if you have coax or phone wire running through your walls and your whole-home coverage is poor, wire it. MoCA adapters turn existing coax into gigabit ethernet runs. $150 for a pair gets you a reliable backhaul to a second access point at the other end of the house. Wireless everything is a dream; wires are reality.