If the modem/router is delivered/sold by the provider and they deny support, they might lose a customer next time
Anyway, I have no clue about this but you can get alternative FW for WLAN-routers. Not sure if this is the case for yours, but the projects have usually listed models of routers that are supported iirc. I tried once DD-WRT with my Asus WLAN-Router and it worked well.
Check
Wikipedia for a list of available projects.
Though keep in mind, you can trash your router with it, if the update fails.