How to configutre LTE Router with passthrough

Hi,
hope you are all fine. I wonder if someone can give me good advice. I have a chateau 5g router, which is placed on my roof.
My second router in the cellar is a RB3011UiAS.

First of all I would like to know where I can configure the APN (need a fixed official IP Adress).

And then I would like that the RB3011 will get this official IP Adress.
I was using a Zyxel NR7101 Router and IP passthrough was the standard setting.

So all in all my LTE router should work as a normal modem brindge.

Thanbks in advance for your help.
Regards.
Arthur Mössmer

Shouldn’t be too different from:
http://forum.mikrotik.com/t/uhg-lte6-ip-passthrough/141183/1