Hi,
If I understand correctly, both UK SIMs work correctly without issues, but there is an issue with the Free SIM card, which works only on a first SIM slot? Have you tried to change SIM interface settings (APN, authentication)? It may be that they require specific APN to work. Also, which modem firmware version is your router using right now? You can find that out by navigating the System > Firmware page.
Regards.