@Pippagirl - check the Messages settings. iMessage should be enabled (I'm sure it is) as well as Send as SMS and MMS Messaging. WiFi also needs to be enabled. It's not a MicroCell issue as there are no settings to check or modify.
Your phone is capable of WiFi-C (WiFi Calling) so you don't really need the MicroCell. They have reached their EOL as of the end of 2017 and AT&T's 3G service itself will eventually be discontinued as well rendering the MicroCell useless.
Award for Community Excellence 2019 Achiever* *The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.