Is there a reason why I have to have a new router each time I want to change suppliers? What to providers do with the returned ones?
Hi Rachel,
Old routers are often repurposed/refurbished to working order and given to new or existing customers of the ISP they were from. If the routers are end-of-life or beyond repair or not worth refurbing, they’ll have them recycled through a WEEE certified electricals disposal contractor.
The reason you get a new router with each provider is that some providers lock down the configuration of their routers to prevent certain things being changed that might prevent the service from functioning. It seems like a very wasteful regime, however it does prevent a lot of headaches in supporting users where they’ve changed something in the router settings and “broken” their service 
I hope this helps.
-Andrew