They can't use those methods anymore...
As mentioned in the question, we've previously seen two methods of opening the Cage, but neither of them are viable anymore:
The Seals Although there are a total of 600+ seals, only 66 of which needed to be broken to open the Cage, at least the first and last appear to be fixed. The first, that "a righteous man spill blood in hell", doesn't seem terribly difficult to replicate; it's the last, Lilith's death, that's the stickler:
Ruby: And it is written that the first demon shall be the last seal.
Supernatural Season 4 Episode 11: "Lucifer Rising"
Since Sam already killed Lilith, using the seals is out of the question
The Horseman's Rings. Although Sam and Dean have three of the Rings buried in Bobby's yard, it seems exceedingly likely that Death's ring was destroyed along with his scythe when Dean killed him in the season 10 finale
...But they wouldn't be helpful anyway
Even if the other methods were still viable, they present a major risk: Lucifer himself. Neither method guarantees the two things the Brothers want out of the meeting:
- They can talk to Lucifer without him killing either of them
- Lucifer can't escape and restart the Apocalypse
This is really the bigger problem; dialogue from the episode suggests that they're not worried about getting into the Cage, but rather how to safely meet with Lucifer outside the Cage:
Dean: Can Sam meet with Lucifer or not?
Sam: In the Cage.
Dean: No, not in the Cage. That’s not going to happen.
Sam: Okay, then I don’t meet with Lucifer. Dean, we can’t let him out.
Dean: [notices a look on Crowley's face] What?
Crowley: There may be a way. Clearly, if Sam enters the Cage he’s gone. And yes, it’s on my bucket list, now is not the time to be selfish. Need a secure site, a way to neutralise Lucifer’s powers.
Supernatural Season 11 Episode 9: "Oh Brother Where Art Thou?"
The Angelic Elephant in the room
At this point we have to mention how Castiel broke in to retrieve Sam's body. We don't know how he did it. At all. No clue. The production staff seem content to leave that question unanswered for the moment, and nobody on the show has made a big deal about it.
Without knowing more about how he did it, we can't say anything conclusive about why the Brothers didn't try it. It's just a plot hole that seems to have been swept under the rug and then forgotten about.