sorry.. I smile at the "taking a dump"
one of my sayings with regards to the 922 series receiver ... because everyone makes such a rave about the "polished" User Interface ... is that you can spit shine a "turd" but its still a turd..
alright.. back to your issue specifically ..
k... Stability is one key .. I believe I mentioned that above.. the more stable things are there, the more likely to keep online at Dish/Sling/"AmAkie" ('splain that later) As to the Router with DD-WRT ... what build and package? (meaning build # and is it mini, vpn, std, max for the base code type) and what router did you put it on (name and model)
If you're not familiar enough .. DD-WRT is great .... in two ways.. expansion and frustration. DD-WRT gives you great capabilities to do things that normal firmware doesn't let you do (IP Tables is one of them) but at the same time, because they haven't released a "defined-as-stable" build in a long while.. every build is likely to have fixed some things and may have broken others. That's the one thing I've got to say I like least about DD-WRT is the lack of getting a good beta build together for users to hammer on, and then go into release.
The reason I'm concentrating on DD-WRT (asside from having it here to id screen and field locations) is that most of the time its UPnP support works without issue. But given the model & brand of router I'd dig in on that to find out if there are any *don'ts* listed, and at the same time... to have you go into DD-WRT set it up as your gateway, taking care to set up Static DHCP for all of your Dish Receivers ... and then see what happens in 24 hours.
That "stability" is really to give the Dish/Sling/AmAkie servers time to get a lock and cache going.
AmAkie is my "toy" concatenation for the newest way that I've found Dish Online to be functioning, that is your authentication gets handled through the Amazon Cloud, and the Content coming through Akamai proxies. It takes time for them to come up to speed ... and even when they *look like* they know you and your DVR's ... in reality you'll find that its just their cache of information that you're seeing .. an example is to delete something at your DVR with the remote control .. and then check every 30 mins or so to see if the item disappears from your "View DVR" option at Dish Online and Dish Remote Access.
Once you have a well defined connection to the internet, UPnP is out of the way, and your receivers aren't having any chance of changing IP's (because you setup static DCHP) then its just a matter of hitting REFRESH at the respective web page for DRA or DO to have it reach out to your DVR and get the update..
So.. suggestion is to most likely try the DD-WRT router as your gateway router, assign IP's to your receivers (I recommend starting at 192.168.1.250 and coming down 192.168.1.250 / .249 / .248 ...for your first, second, third
sling-able devices) and then test the internet from the receiver (free on-demand download like a music video) and then confirm in DD-WRT's NAT/QoS -> UPnP screen that 5101/5102/5103... have been grabbed and assigned your respective receivers ... also recommend giving simple names to the boxes .. I use the family member's name T for me, CJ for my sister, LR for living room so that you can easily see which is which.. etc..
and in RE 20 unit apartment complex.. I'd bet your on 3 phase. look for your incoming power poles .. if you see at least 3 big lines going down the pole to a pipe (or to the side of the building) you're more than likely on 3 phase.