[Synthetic]

TP-Link needs to be in router mode, SH in modem mode.

WAN connection setting needs to be DHCP, you also need DHCP enabled on the LAN side but that should be set as default anyway.

Make sure you have the your cable from the WAN port on the TP-Link into the port nearest the coax on the SH.

My TP-Link is the VDSL version but the setting for the WAN is "Internet Connection Type" = Dynamic IP for DHCP ISPS, which Virgin will be.

EDIT: Also give your SH a reboot after you've set the tp-link up, i had to do this for my pfsense box to get an IP, not sure why, or if it'll be the same here but worth a shot