An ISP service guy split my WiFi router signal into regular and "5G", but now neither works well. How do I switch it back?

Let's try this again. As David mentioned, your Wi-Fi router has the ability to send and receive on 2 frequencies: 2.4 GHz and 5.0 GHz. That's not a bug... it's a feature. There's nothing to "switch back." The 5 GHz freq. is new in town so you'll probably only have a few devices that can use it.

Many things affect Wi-Fi. You should do some Yahoo Answers searches and see what others have done.

As far as things not working well -- have you tried rebooting your router? If so, what happened?

You don't split the signal, your router must have the ability to turn off the 5GHz band. Log into the router and there will be a setting somewhere