Do you mean you want it to be part of the existing network on another modem? (Then read the manual, router as IP client page 78 https://assets.avm.de/files/docs/fritzbox/fritzbox-7590/fritzbox-7590_man_en_GB.pdf)
Or you want it to act in bridge mode (another router will take care of the routing?)? Then I don't see that being available.
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