The WiFi modem/routers that are typically supplied from internet providers are pretty basic, and pretty weak. There are many things that can cause interference and speed issues, some may be in your house, or possibly coming from the neighbors house. Also any wall, ceiling, floor that the signal goes through weakens and slows it down.
Test your speed with a Ethernet cable plugged into your modem. If you are getting the correct speed there, it is a WiFi issue, and not really anything your provider can fix.
If it is WiFi issues, look into a Mesh network router, they are made by most router companies and they provide tremendously better performance. They actively monitor for interference and change frequencies/channels automatically. Also the use multiple units spaced around the house to cover the house evenly to eliminate weak/dead spots.
The downside to these mesh routers is they are expensive ($200-300). I use one made by Eero, and it has increase my speed in every area of my house, where I used to get 5-40mbps in many areas of the house, now I get 145-148mbps throughout the house.