A proxy that will not connect: what to check first
Check the password for stray spaces, make sure the browser got HTTP rather than SOCKS, and do not fire a second IP change while the first is still running.
When a page spins or the browser says the proxy refused the connection, the modem is often fine. The setup is not. Go through these before you assume the line is dead.
The copy
Paste the username, password, host, and port again from the provider’s dashboard. Do not trim by eye. A single space before the password makes the login look wrong forever. If the browser offers to save the old password, clear that saved login so it does not keep submitting the broken one.
The protocol
Browsers need the HTTP endpoint. Apps that say SOCKS5 need the SOCKS endpoint. Swapping them produces an instant failure that looks like an outage. The password will not fix a crossed port.
A change already in progress
If you just asked for a new IP, wait. On a mobile line that can take a couple of minutes, and a second click while the first request is running often leaves the session stuck. Check the IP once, after the dashboard says the request finished, not every thirty seconds.
The network in front of you
Some office and public networks block outbound proxy ports. Try the same browser settings on a phone hotspot. If it works there and fails on the other network, the proxy is up and the network is filtering it.
If the copy is clean, the protocol matches, no IP change is in progress, and a second network fails the same way, write to the provider with the exact error text and which endpoint you used. That is enough for them to see the difference between a bad password and a down modem.