Fanvil i63 - Assign Ext greyed out

[HopeMedia]

Self hosted 3CX Environment -
Version - 18.0 (Build 461)
Phone - Fanvil i63 door phone (listed here as supported https://www.3cx.com/sip-phones/fanvil-sip-doorphones/)
Phone firmware - 2.4.4 (supported firmware as listed here - https://www.3cx.com/support/phone-firmwares/)

I've got it registering with the PBX and showing up as a new phone.
If I click on the select check box , "assign extension" and "new extension" are greyed out.

Any tips for getting it registered against an extension?
I bought the video phone as it had five speed dials and was listed as a supported device on the website.

 

[JohnS_3CX]

Please update your PBX so you can get the latest supported devices.

You are on update 3, but our latest is update 5 now.