The instructions state "of your other / existing router", i.e.: main router of the third-party provider, in your case "Salt Fiber Box".
Setting up FRITZ!Box as an IP client
https://avm.de/service...
When operating as an IP client, the FRITZ!Box uses the existing Internet connection of your other router. In this case
the firewall of the FRITZ!Box is deactivated.
the computers, smartphones and other devices connected to the FRITZ!Box continue to receive their IP settings from the existing router.
All devices can access each other without restriction, regardless of whether they are connected to the FRITZ!Box or to the other router.
Aside:
According to the descriptions in the various AMV Fritz! products, MESH is only possible with Fritz! models that have FRITZ!OS 7 (and higher) installed.
Example:
The FRITZ!Repeater 3000 has two types:
Variant Set up LAN bridge
Activate MESH variant (only with FRITZ!Box from FRITZ!OS 7)
Page 21:
Requirements for operation
For operation with LAN cable: Router with free LAN port [router is not specified in more detail].
For Mesh: FRITZ!Box with FRITZ!OS 7.00 or newer
https://assets.avm.de/files...
P.S.:
At AVM you have to search in the downloads and in the knowledge base.
If something doesn't work, then Salt customer service.