I get r/ds pretty frequently, but this is the first time it says Unable to Connect and it won't let me get on or anything, even though clearly my internet is working fine.

What does this mean and what can I do to get around it?

Take a look at this guide: http://forums.d2jsp.org/topic.php?t=39461179&f=21

If it doesn't help you, then you must wait them out

In the future don't join and leave games to quick and you won't evet get a realm down

Usually you just wait 5 minutes but you can use the same process you do for bypassing a realm down as foe UTC

http://forums.d2jsp.org/topic.php?t=39461179&f=21