How to test a proxy before you rely on it
A useful test is short: confirm the address, see whether it moves on its own, and only then sign into something you can afford to lose.
A proxy can connect and still be the wrong tool. The test is not “does a page load.” The test is “is this the address I think it is, and does it stay mine.”
Confirm the exit
With the proxy on, open a page that prints your public IP. It should not be your home or office address. Write it down. Turn the proxy off and reload. The address should change back. If it does not, the browser is not actually using the proxy.
See if the address is stable
Leave the session alone for a while. Browse a few ordinary sites. Check the IP again. If it changed and you did not request a new one, you are on a rotating or shared pool. That can be fine for some jobs. It is a bad surprise if you needed the same mobile address for a login.
If the provider lets you request a new IP, do it once and wait until the request finishes. Then check the address again. You want to see that a request you made is what moved it.
Use a login you can throw away
Only after the IP behaves, sign into a test account or a low-value profile. Do not start with the account you cannot replace. Watch for an instant lock, a challenge you never saw on your normal connection, or a session that dies when the IP flips.
If the test login is boring and stable, the line is worth using for real work. If it is chaotic, another day on the same pool will not fix the sharing.