No, there is no obligation for mandating a lawyer in case your EU trademark is being infringed. However, instructing a lawyer can make sense. A warning letter is a sensitive issue. Above all, it is the basis for any further dispute, including court action. Any mistake in the request for a cease-and-desist undertakting can have serious consequences. In addition, sending off an unjustified warning letter can have serious consequences. Therefore, it is strongly advised that a qualified lawyer or attorney reviews the matter and also represents you vis-à-vis the opponent.