This assumes the extender and router have the same network name.
I would actually set the static IP address in the router and not on the Pi in this case as I have had odd issues with the BT extenders and my daughters flat before now (You will see lots of posts for me arguing to set the IP on the Pi normally
You will have to log on to your router and find out how to set the reservation based on the MAC address of the Pi and this can be found by the terminal command
This will return lots of data and you need to find the part that looks like:
wlan0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.1.212 netmask 255.255.255.0 broadcast 192.168.1.255
inet6 fe80::4d89:db63:c5d8:1607 prefixlen 64 scopeid 0x20<link>
txqueuelen 1000 (Ethernet)
RX packets 24774 bytes 4473962 (4.2 MiB)
RX errors 0 dropped 7 overruns 0 frame 0
TX packets 11583 bytes 6231015 (5.9 MiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
The MAC address is in bold/italic above (line starts ether) and will be different for your Pi. This is from a Zero W and has the MAC of b8:27:eb:85:a6:5d
You may need to do this for both IPv4 and IPv6 depending on the area of the UK you live in and if your line has IPv6 enabled or not...