SMS messages not syncing with Messages on High Sierra

Since upgrading my iMac to High Sierra, Messages has stopped picking up or enabling me to write SMS messages. It still handles iMessages with no problems, but this means that when communicating with phones that are not iPhones I need to read and write on my iPhone 7 itself, or on my MacBook Air, which still runs El Capitan, and is still able to sync as before. I have compared settings in both the Messages app and for iCloud and they are identical between the iMac and the MBA. Any advice on how to resolve this?