More information needed.
Update your post to include full system hardware specs and OS information.
Make and model modem?
Make and model router (or modem/router if combined)?
Who provides the modem and/or router - do they belong to the ISP or to you?
How is everthing connected? Where is the switch located?
Normal/expected network connectivity - line diagram (where ----> represents an Ethernet cable):
ISP === coax, DSL, fiber ===> Modem ----->[WAN port] Router [LAN Ports] -----> wired devices (can include a switch) plus ~~~~ > to wireless devices.
Feel free to edit and correct my line diagram as necessary.
Add some explanation about the specific wireless problems that are occuring.