Open the forum from the browser of your phone.
Near the upper right hand corner you will see 3 horizontal dots.
(next to the magnifying glass)
When you click on those 3 dots you will see the option to sign in.
You will also see an option to change your settings, such as increasing font size if needed or choosing if you see thumbnail images or not.
Near the upper left hand corner you will see 3 horizontal lines.
Clicking there will offer more navigation and options.
If you want to reply to a thread, simply click reply at the end of the page or click the + sign near the top.