There are some great apps like Signal but also peer-to-peer apps like Briar, Manyverse, Jami and others which communicate directly across Bluetooth and/or Wifi without the Internet.
If the Google Play Store is censored or banned for you there are alternatives like F-Droid and other stores to install form. But why wait until there is crackdown - install two or three alternative messenger apps now and be setup. These can also be used for community emergencies during loss of Internet for natural reasons.
See Hong Kong downloads of Signal surge as residents fear crackdown
A new security law is expected to undermine Hong Kong’s autonomy and freedoms.