Hi Clive,
Why you static route on SRX1500?
SRX1500 should advertise the subnet via ISIS and LNS should have isis route for 2a05:d840:0050::/48. SRX1500 and radius is P2P. All you need is return route from radius.
Can you remove the static route and try to ping SRX1500 [ge-0/0/2] from LNS? In case of any issues, please provide the output of show route from SRX1500 and LNS.
Regards,
Rahul N