Can you make and receive phone calls?

Try toggling airplane mode, rebooting the phone, remove/re-insert the SIM, or try the SIM in another phone as well.

Am I suppose to use my old sim card or the new one

Am I suppose to use old sim card or new one?

You should be able to continue to use the old SIM card, as long as it fits in the new phone.

You can also use a new SIM if you like, but it’s not required. If you do, you’ll have to activate / switch to the new SIM card by entering its IMEI code in your self-serve account.

If you already put the new SIM into the new phone, follow this to switch your SIM https://www.koodomobile.com/en/help/koodo-sim-support