Market participants shall register with the National Regulatory Authority (NRA) in the Member State in which they are established or resident or, if they are not established or resident in the Union, in a Member State in which they are active as the NRAs are handling the registration process. NRA’s will transmit the information in their national registers to the Agency. Each Market Participant can register with only one NRA.
The list of NRAs with contact details can be found here https://www.acer-remit.eu/portal/ceremp.