The Cisco Webex Room Kits do support 802.1x, so they should be able to connect to a Meraki SSID using 802.1x. However the issue will be whether there is a supported EAP type that both the Webex device and Meraki authentication have in common. It looks like they should (Webex device support EAP-MSCHAPv2, EAP-TTLS and PEAP, https://www.cisco.com/c/dam/en/us/td/docs/telepresence/endpoint/ce912/room-kit-administrator-guide-c... page 22) so you probably just need to add the username and password to the Webex device - I can’t recall if I’ve done this before because mostly I connect them by cable.
I’d be surprised if the Webex device didn’t already have the correct CA installed by default, or you can find the xCommand to disable certificate verification (not ideal, but could prove if the certificate is the issue).
EDIT: check the details for xCommand Network [1..1] IEEE8021X TlsVerify
