Here's things that have worked for me:
- Write something funny or stupid in response to the update (especially if it's high drama).
Example: I wrote a chapter where a character was basically hiding in a cupboard having a massive internal drama and thinking of himself as a rat clinging to a sinking ship. The episode note just read "Squeak squeak motherfucker." I got a lot of people responding things like "OMG the authors note XD"
- Ask a question or poll - especially if it's something shipping or theory related.
Examples: For The Mortician of Avalon, I ask people what their theories are and encourage people to guess things around some of the central mysteries (identity of the killer, whether or not one of my MCs is a bad guy, just what is the mysterious dog that seems far too intelligent to just be a dog etc.). On Freedom's Pyre, I asked a shipping question. "If LOVE INTEREST wasn't in the plot, who would you ship MC with?" That got lots of engagement.
- This one is easier in novels but - decent cliffhangers. By this I don't mean ones that just end at high drama (readers soon get annoyed at that) but having a strong degree of 'what is going to happen next????' at the end.
Example: I just ended a chapter of Freedom's Pyre with a character that everyone thought was a true ally of my MC declining to be in his political alliance. I haven't given his reasoning for it yet. The chapter has a resolution (i.e. the main question of the chapter has been answered) but I've not given them the resolution they want yet (i.e. the reason he's declined) which has led to speculation. This is harder to do in webcomics when you upload maybe a page at a time though, admittedly.
- Links to extra content that has been created but is hosted elsewhere
Example: Freebies from Patreon, or one time I made an advent calendar of flash fiction. That got people commenting to say thank you for providing extra content for free.
- Set up a Discord, add people. Respond to every comment, even if you don't know what to say, an emoji still works. Engage the commenters you do get more deeply by asking them questions to draw out responses. (Don't think I need an example for this one). People are often scared of being the 'first' or 'only' person in the conversation. Encourage them to also chat to each other.
Hope this helps?