One of the first issues is 2.4G only has (11 channels) only 3 usable (1, 6, 11) which your wireless can be set to. The problem starts with your neighbors. They're also sharing those 3 channels. That causes interference and retries which slows speeds. 2.4G does have some advantages though. First is range. You will likely get one and a half the range out of a 2.4G WiFi network compared to a 5G network. The second advantage is compatibility. Some legacy devices will only work on 2.4G.
5G on the other hand is faster, has more channels, which means less interference. If your device supports it, always use 5G.
I also wouldnt judge your actual speed off wireless. I would hardwire to the router, then do a speed test. If the test is still slow then you are having an actual connection issue. It is really difficult to achieve gigabit speeds over wifi. Wireless isn't meant for speed, it's meant for convenience.