In planes and public transportation are generally medical mask, KN95 oder FFP2 mask required. A mask made from cotton or cloth are not allowed. Lufthansa actually enforces this and I also noticed this in trains. Hotels and Shops vary, but with a medical mask, , KN95 or FFP2 you are on the safe side.

For travel on Deutsche Bahn see https://www.bahn.com/en/booking-information/corona with additional information. The local transport in Berlin (noticed you asked in a different thread) ask for FFP2 (see https://go.bvg.de/corona only in German). I left Berlin today, and also saw people with a medical mask and KN95 without problems.

Iserlohn belongs to North Rhine-Westphalia. And the recent Corona Regulation requires FFP2 mask in local transportations. In Iserlohn this would be "MVG Märkische Verkehrsgesellschaft" (https://www.mvg-online.de/faqs - also only in German).