Okay, here is the definitive answer for connecting a 27mm hose to a Dewalt 611 trim router (corded or cordless makes no difference, they both use the same dust collection accessories).
Below we have the Dewalt DWV9120 and Bosch VAC003. Both options work, the Dewalt is around $15 the Bosch is around $5. And in this case I think the cheaper Bosch option is the best.
Bosch VAC003 - this one is hard plastic, fairly stubby and fits all the way down into the dust collection fitting. The inner collar in the dust collection fitting is tapered, so the fit is tight as you seat the Bosch adapter all the way in. The Festool 27mm hose fits perfectly into the Bosch adapter. It seems like it would take a bit of force to undo either connection.
Dewalt DWV9120 - this one is a two part adapter. The rubber part fits into the dust collection fitting but it doesn’t bottom out like the Bosch does. Luckily it’s rubber, so friction keeps it in place pretty well. The junction between the two parts swivels, but if you’re using a Festool hose, it already swivels, so this feature is moot. The second part of the adapter is plastic and the 27mm Festool hose fits into it perfectly. While this adapter appears to work well, my guess is this one would break free easier than the Bosch. This adapter is longer and I suspect that might apply enough torque to the friction fit between the dust collection fitting and rubber part of the adapter. As you can see from the picture, it’s not seated very far into the fitting.
One more note comparing the rubber part of the Dewalt adapter to the Festool hose. They both have the same outer diameter, however, the Dewalt has a larger inner diameter. When you plug the Festool hose directly into the dust collection fitting on the router, it stretches over the inner collar in the dust collection fitting enough that the outer diameter will no longer fit. Maybe you could force it, but I suspect you would break the dust collection fitting in short order.