I updated the firmware to Lede testing 2.27. Modified the network manually to following (using ssh),
config interface 'modem'
option service 'umts'
option apn 'giffgaff.com'
option metric '40'
option country 'country71'
option sp 'isp4'
option dialnum '*99#'
option username 'giffgaff'
option password 'passowrd'
option proto 'ncm'
option ifname 'wwan0'
option device '/dev/ttyUSB1'The device now comes up,
Sun Jul 22 02:23:12 2018 daemon.notice netifd: Interface 'modem' is setting up now
Sun Jul 22 02:23:14 2018 daemon.notice netifd: modem (2347): sending -> AT
Sun Jul 22 02:23:15 2018 daemon.notice netifd: modem (2347): sending -> ATZ
Sun Jul 22 02:23:15 2018 daemon.notice netifd: modem (2347): sending -> ATQ0
Sun Jul 22 02:23:16 2018 daemon.notice netifd: modem (2347): sending -> ATV1
Sun Jul 22 02:23:17 2018 daemon.notice netifd: modem (2347): sending -> ATE1
Sun Jul 22 02:23:17 2018 daemon.notice netifd: modem (2347): sending -> ATS0=0
Sun Jul 22 02:23:18 2018 daemon.notice netifd: modem (2347): sending -> AT+CGDCONT=1,"IP","giffgaff.com"
Sun Jul 22 02:23:19 2018 daemon.notice netifd: modem (2347): Starting network modem
Sun Jul 22 02:23:19 2018 daemon.notice netifd: modem (2347): sending -> AT^NDISDUP=1,1,"giffgaff.com","giffgaff","passowrd"
Sun Jul 22 02:23:20 2018 daemon.notice netifd: modem (2347): Setting up wwan0
Sun Jul 22 02:23:20 2018 daemon.notice netifd: Interface 'modem' is now up
Sun Jul 22 02:23:20 2018 daemon.notice netifd: Network device 'wwan0' link is up
Sun Jul 22 02:23:20 2018 daemon.notice netifd: Network alias 'wwan0' link is up
Sun Jul 22 02:23:20 2018 daemon.notice netifd: Interface 'modem_4' is enabled
Sun Jul 22 02:23:20 2018 daemon.notice netifd: Interface 'modem_4' has link connectivity
Sun Jul 22 02:23:20 2018 daemon.notice netifd: Interface 'modem_4' is setting up now
Sun Jul 22 02:23:20 2018 daemon.notice netifd: modem (2347): Command failed: Unknown error
Sun Jul 22 02:23:21 2018 daemon.notice netifd: modem_4 (2396): udhcpc: started, v1.25.1
Sun Jul 22 02:23:21 2018 daemon.notice netifd: modem_4 (2396): udhcpc: sending discover
Sun Jul 22 02:23:21 2018 daemon.info odhcpd[1220]: Using a RA lifetime of 0 seconds on br-lan
Sun Jul 22 02:23:24 2018 daemon.notice netifd: modem_4 (2396): udhcpc: sending discover
Sun Jul 22 02:23:24 2018 daemon.notice netifd: modem_4 (2396): udhcpc: sending select for 10.147.143.192
Sun Jul 22 02:23:24 2018 daemon.notice netifd: modem_4 (2396): udhcpc: lease of 10.147.143.192 obtained, lease time 518400
Sun Jul 22 02:23:24 2018 daemon.notice netifd: Interface 'modem_4' is now up