There is currently still a template for them in 3CX however that model does reside in the EOL section for phone support: https://www.3cx.com/sip-phones/
Only fully supported/preferred phones qualify for this setting normally, however I would confirm your firmware version and if possible upgrade, re-provision and try again.
Please note that these models do not support uaCSTA which is what the web client is using to "control" the IP phones so End of life models are not recognised by the web client. You can see a list of limitations for these phones in the guide below: https://www.3cx.com/sip-phones/yealink-t20p-t22p-t26p-t28p/#h.e2e9dlym4hi8