On a Roamio, there's 2 ways to delete with no internet.
1) Fast forward to end of show, delete using window that pops up.
2) From show list, press Clear button to delete show.
We discovered this because there are so many problems with connectivity when using the built-in wifi. We get Roamio "internet errors" probably at least once a day, when all other wifi devices in the same area work perfectly. We'll likely plug the Roamio into a wifi extender eventually because the built-in wifi is so unreliable.