There are various ways to keep track of new activity on these forums. Simplest of course is to keep visiting the web page, or keep a tab open. You can even enable desktop notifications.
On mobile, I’m not sure what the story is, so perhaps others can add their findings. If you don’t visit for a bit, the forum will send you a ‘digest’ email with selections from new posts - you can configure this in your account settings, for example choosing daily or weekly digests.
I’m most at home in email, so I like to use the ‘tracking’ feature which sends me emails when things happen. You can track specific conversations, or categories. I’ve got all categories tracked now, so any new post will send me an email, if I’m not presently active on the website. Again, visit your settings or visit each category and use the gadget at top right to adjust the notification level.
I also use an RSS reader for keeping up with lots of websites. There are feeds for each category, or global feeds:
(Some email programs have an RSS reader built in. I use a website, https://www.inoreader.com/ - others are available, including https://theoldreader.com/)