Public Mobile is prepaid. You should do one or the other but not both at the same time.
If your plan has been expired for less than 90 days, you can logon your account to make a payment or call 611 to make a payment if your cc is already on file.
If you want to change plan, you can read here: https://www.publicmobile.ca/en/bc/get-help/articles/change-your-plan
If your plan has expired for 90 days, then your old number and plan are gone. You will need to get a new SIM and start a new account/plan.