Try holding down the Bluetooth button on the SoundCore Sport XL for 2 seconds to unpair it from the last device. The speaker should then go back to pairing mode. Go back to your iPhone (Settings > Bluetooth) and the speaker should show up again.
If you think one of your other devices is interfering, then you may want to try taking your speaker and iPhone to another area. I once had to take one of my Bluetooth speakers to a local park because I couldn't figure out which device in my house it kept connecting to.
If it still gives you trouble, feel free to reach out to firstname.lastname@example.org.