As Koodo hasn’t  supported 5G yet, I suggest you turn off 5G and default the phone to LTE/HSPA. That might help with the battery and signal. 
As all newer  Samsung phone model supported RCS. RCS also request a data connection. The issue could be from that also. Can you try to turn off RCS in the phone and see if it helps?https://www.koodomobile.com/en/help/next-generation-messaging

Thank you for your reply.  I have turned off chat on my messages but you mentioned switching to LTE/HPSA.  My phone only has LTE/WCDMA/GSM (auto connect), WCDMA/GSM (auto connect), WCDMS only and GSM only.  Could you tell me which of these would work to turn off 5G?

If you didn’t see the 5G, your phone might run the Koodo firmware already. As such, you don’t have to do anything. 5G was already off.