In order for messages to sync correctly on an iOS device, iMessage must be shut off.
One of the most effective ways to do this, which often results in it being deregistered a little faster, is going to the following website: Deregister and Turn Off iMessage - Apple Support
Apple indicates it can take up to 24 hours for them to completely deregister you from iMessage so there is definitely a lag time from when you turn off the service before all of your contacts who have iPhones are updated.
This should allow your message to send and receive as normal SMS. Please let us know if you have further issues after 24 hours from taking that step.
I’ve already completely deregistered.
Just saw your 2nd email, so, I hope the next update fixes things
Sorry that you're having an issue here. If you had updated to the latest client as of a couple of days ago, we're having some intermittent issues with messaging on iOS. The replies 'should' be going to all devices that are registered with that number, from the SIM'd device to any of the clients. It sounds like the DIGITS app on iOS isn't getting it, but you are getting it on at least one other device.
I know a fix went in last night that cleared up some issues for me, would be good to know if you're still having the issue today, and if so - please let us know and we'll get one of the forum admins to collect additional details from you.
Thanks for trying out DIGITS and providing us with valuable feedback.
Yes! It started working after a few days of me not really doing anything
besides staying signed in.