What is the difference between a router of 40mHz and 20mHz?

Both 40mhz and 20mhz are the current wireless signal bandwidth in wireless local area network technology. In theory, 40Mhz has twice the channel bandwidth compared to 20Mhz, and the theoretical data transmission rate is also twice as high; However, from a practical application perspective, setting a higher bandwidth has a greater chance of conflicting with surrounding WiFi signals, which can affect the actual data transmission rate. 4G Cat4 Cpe

The difference between 20Mhz and 40Mhz can be imagined as the width of the road. The wider the width, of course, the more data you can run, which increases your speed. But the "road" of wireless networks is shared by everyone. As you occupy a wider road and run more data, it is easier to collide with others. Once you collide, everyone will slow down, even slower than walking on narrow roads. Please refer to this figure:

Originally, squeezing at 20mHz could allow four people to use it at the same time, but if you use 40MHz, you can only use it for two people at the same time. So the question of which one is better is not related to the size of your house, but rather to how many people are traveling with you nearby. Using scanning software can easily see the occupation of surrounding frequency bands.
So, Beijing Broadband Connect suggests that if there aren't many people nearby using it, congratulations and enjoy the highway with 40Mhz! If there are many "vehicles" around you, it is best to find a "lane" with fewer vehicles, and it is better to use 20Mhz. 4G CAT6 CPE

In addition, 40MHz requires the support of the customer terminal, otherwise it will still fall back to 20MHz mode. Older phones and laptops, as well as cheaper smart home products, do not support 40MHz. 5G CPE
Moreover, the 2.4GHz total frequency band is very narrow, with a total of 14 channels compared to 13 channels in China, and some devices only support 11 channels. One WiFi occupies two channels and has significant interference, so it is generally recommended to use 20MHz for 2.4GHz WiFi; 5GHz WiFi can be used at 80MHz or 40MHz. For bridging, it is recommended that both the main router and the bridging router use 2.4GHz WiFi, 20MHz mode, and use the same channel.