Classic "Invalid SIM" and "SIM not supported" messages almost always mean the phone is locked. By whatever carrier originally sold the phone brand new out of the box.
Such a phone will only work with SIM cards issued by that carrier. Unless they unlock it or you buy an unlock for it.
If the phone was locked by Telus or Koodo then Public Mobile can unlock it, I think.
Once this is completed and you paid for a new plan, insert the SIM card and restart your phone.
If you have any issues, can you try your SIM card into another compatible device to see if services work?
If services still do not work, submit a ticket to CSA for assistance.
To contact the Public Mobile Customer Support Agent (CSA)_Team, there are two methods to reach them found here: