There's a few ways you can subscribe based on your preferred approach.

Examples:

• if you are on the community you want to subscribe to, tap the three dots button and tap subscribe
• if you are on the community you want to subscribe to, tap the toolbar to open the community selector. The community selector has a section that shows which community you are currenty browsing. There is a subscribe button there.
• if you see a post from a community you want to subscribe to, you can long tap the community name and tap subscribe.
• you can also enable post feed headers which makes the subscribe button even more prominent. To enable this setting go to: Settings > Post feed > Turn on Use posts feed header.

Let me know if you have any other questions.