So, you want the first Chatr phone number to be assigned to your second newly activated Public Mobile sim card so that you can recover your lost phone number, for the sole purpose of being able to port it back to Chatr. Is that correct? Ok, so your first step, you've got right; you need to get the phone number back first. You will have to get Public to go back into the phone number aging pool and have it assigned to your most recently activated account.
After that, you can definiately port it back to Chatr. Howver, the second you ported it from Chtar to Public, that would have closed out your Chatr account. You'll need a new Chatr sim card and you'll need to pay for a new Chtar plan. Whether Chtar can/will credit you for an unused portion of your old Chtar plan is up to them.
