I'm assuming you talking about a modem attached to your home phone line, that then provides wireless to your devices at home, rather than a usb wireless modem that connects via mobile phone services to provide data to your computer. 

Do you live in an area with either increasing population and/or increasing takeup of DSL services?  Is it holiday season, with more people at home using their internet? That can mean more demand on existing services and a corresponding reduction in speed. Other possibilities include something affecting the phone line between the exchange and your home. Has there been rain? Maybe a pit somewhere is full of water.

And, are you referring to the mobile devices connecting to your wireless modem at home, or their connection to the telco? If the former, it does sound like the modem is the problem.

I have a Thompson wireless modem and I had to give all my devices fixed ip addresses to prevent it dropping out the third device to connect, anytime. It is still quite unstable when the devices connecting to it are changed, so I also restart it on a regular basis. 

I hope this was of some assistance.