Hmm, odd - they're present and correct here.
This discounts their somewhat erratic appearance though: sometimes a thread shows no new posts but the date/times of the last post column on the right hand side often shows that to be a lie!
Neither your modem nor computer would be responsible for this.
Basically, if you have WP displayed in a browser and it looks Ok then everything else should follow.
If you reload a forum index page which had those arrows on it then once the page is redisplayed they will have gone - because they're now no longer new posts.
The reverse is true too: if there were none to start with and on reloading the page some appear, those will be new posts made since the last page reload.
Aside from WP occasionally working to its own odd ideas about what's new and what isn't, it should be working Ok.
It's a bit vague I'm afraid - but try logging out of WP completely, close and restart the browser, then log into WP again.
Do nothing with it for (say) 15 minutes then open up a forum you know to be busy and is certain to have new posts made within the time you waited, and see what's displayed.
PS - apologies for the delay in responding!
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Giraffe: a ruminant with a view.