Connected usb drive

> How to give static ip [address] to my usb drive on my network? [...]

      https://en.wikipedia.org/wiki/Internet_Protocol
      https://en.wikipedia.org/wiki/IP_address

   Terminology: A "static" address is configured on the device itself.
What you configure on a (DHCP server on a) router is a reserved dynamic
address, not a static address.  Either one should fix the address of a
device, but some implications are different.

   A USB storage device does not have an IP address.

> [...] It is connected to my router (usb 3.0). [...]

   Your "my router" should have a LAN IP address.

> Model: R6400|AC1750 Smart WiFi Router

   That's your "my router"?  R6400[v1] or R6400v2?  Look for "Model" on
the product label.

> [...] the router is access point mode, as it is connected to my main
> basic optic fibre router given by my network provider.

   When you configure an R6400[vX] as a wireless access point, it
should, by default, get a new/different LAN IP address from your main
router, using DHCP.  If you want it to get the same IP address every
time, then you'd probably want to arrange an address reservation (with
the DHCP server) in your (unspecified) "my main basic optic fibre router
given by my network provider", about which I know nothing.