Condom broke and ejaculation inside. Took Plan B.

Ella is the only emergency contraceptive pill proven can prevent pregnancy during the 24 hours around ovulation. You can see that on their website. Plan B might, but it's not proven.

Here in the UK, pharmacists are instructed to give Ella, if you are around your fertile days.

I would not advise taking Ella now, as it's relatively new, & there aren't enough studies done to know if taking both close together, may cause them to cancel each other out, so neither prevents pregnancy.

If you're still within 5 days/120 hours of the sexual intercourse, you can have a Paragard IUD fitted by a doctor, which can work to prevent pregnancy. It can also remain in place to act as longer term contraception.
If Plan B fails, the Paragard IUD can stop the pregnancy from implanting.

Very thick white cervical mucus is not fertile mucus. If that's what you mean. Fertile cervical mucus is clear, like uncooked egg white. You typically get that mucus for a few days before ovulation, when it stays clear & like uncooked egg white, but gets harder to pull apart between your fingers; really gummy. Still clear, though.

Unless you use an ovulation tester kit, it's hard to know exactly when you're ovulating, as ovulation, itself, just lasts for a few hours during your fertile days.

At this point, the IUD I mentioned is the only thing extra you can do, to try to prevent pregnancy.

There are things you can do to stop condoms breaking. It's worth researching. Things like nipping shut a couple of centimeters at the end of the condom, as you roll it onto the penis, making sure no air gets trapped inside it. This leaves an air-free pouch for him to ejaculate into, so it's less likely to burst.