pmerlin:
The electrical meter is the end device
the MKRWAN 1300 i want to use as the "server"
I'm sorry, but IMHO you're just trying to do it wrongly. If the meter is the end device and something is the gateway, you need a Network Server. As far as I know, if you're using LoRaWAN it's mandatory to have one (or more) gateway(s) and one Network server, this is the very basic. The entity responsible to deal with JoinRequest messages and check the keys and generate and dispatch JoinAccept messages is the Network Server. Also, in my opinion, MKRWAN1300 is probably one of the worst options to build a Network server.
The other option would be to use raw LoRa! Some kind of M2M protocol, in order to receive the meter's LoRa messages directly by your MKRWAN. But in this case you'll have to make the meter to comply with this and looks like you can't change the meter, so... Good luck!