Sounds like it’s an issue with your router configuration and NOT your steam deck. If you have tethering on your cell phone you can use it temporarily to test if you get Wi-Fi that way, though I wouldn’t download anything over it if you have a data cap.

Since it sounds like your ISP has complete control of your router I would call their customer support and let them know you have a device that cannot connect. Most likely they’ll either do something like a factory reset on the router or ask for specific hardware information about the device (like the MAC address) so they can check the setting and ensure it isn’t blacklisted or something.