Unable to sync iPhone messages with iMac messages

This just started happening yesterday where if I type a message on my Mac, it doesn't show up on my iPhone in messages. I checked Settings in my phone to have the the send and receive attached to my phone number. Solutions on this forum list iCloud, but I really don't use iCloud and I'm not well versed in technology. I have an iPhone 8 that I don't want to update because I don't want it to mess anything up and bog down the phone...don't know if that's a concern or not. Forgive my lack of knowledge. Can anyone help?