amksd19we Welcome to the community!
We will be happy to answer your questions as follows:
Use the second sunrise internet box as access point wired by cable to the first one?
I would not do this and whether it works I can not tell you.
Use the second sunrise internet box as repeater?
I would not do this and whether it works I can not tell you.
In a mesh?
That’s exactly the kind of system I would build in your house (mesh). For this we have the Connect Pods.
There exists a glass fibre connection between first and ground floor. Is it possible to connect the two boxes via the glass fibre and a “glass fibre ethernet converter” and then put the glass fibre in the WAN port of the first box? Does that work? Would be the best option.
I do not think we will support this. You can get more information from the Fibre Team on 0800 707 707
Greetings
Daniele