If you're getting an error, try using another browser like Firefox, Chrome or Safari.
Also clearing cache/deleting cookies and incognito mode can help.
I switched phone last week, and tried resending the APN parameters. I tried a few times, and the texts didn't come through, so I ended up entering it manually. It took a couple of minutes. The next day, I got 6 texts from Fizz exactly at 4pm.