To link an E top-up card to your device, try texting 150 with ETU then a space and then the 13-digit E top-up card number (the space in between is important so be sure to include it), e.g. ETU 1234567891234 from your SIM.
I'm not sure this overrides an existing registered top-up card to another no. but is only for fresh top-up cards. If it doesn't work, you will need to get a new top-up card from an EE store.