All support for Giffgaff is forum based. Positive Tp-link Archer MR600 result:
https://community.giffgaff.com/d/33163042-giffgaff-sim-in-a-4g-router/9
...but not with the Tp-link-6400:
https://community.giffgaff.com/d/34009104-giffgaff-sim-in-tp-link-6400-4g-router-not-working/11
For me it was a small low risk gamble with Smarty. I simply would have used £20 unlimited data on some other device for a month, and got refunded for the router from the Cex shop if some other network SIM didn't work.
https://community.giffgaff.com/d/33163042-giffgaff-sim-in-a-4g-router/9
...but not with the Tp-link-6400:
https://community.giffgaff.com/d/34009104-giffgaff-sim-in-tp-link-6400-4g-router-not-working/11
For me it was a small low risk gamble with Smarty. I simply would have used £20 unlimited data on some other device for a month, and got refunded for the router from the Cex shop if some other network SIM didn't work.