The library versions of libxDot and mbed-os must match. If you could provide any version information of what you are building I may be able to provide more insight.
However, the AT firmware built online will not run on xDot, the source is provided as an example. There is a difference in the online build environment using a pre-compiled binary that we is causing issues.
xDot AT Firmware binaries are provided here.
http://www.multitech.net/developer/downloads/
This worked to compile with latest. libxDot-mbed5 has an update waiting for testing to complete. Use libxDot-dev-mbed5 with the latest revision of AT Firmware.
https://developer.mbed.org/teams/MultiTech/code/Dot-AT-Firmware/
https://developer.mbed.org/teams/MultiTech/code/libxDot-dev-mbed5/
Update mbed-os to match libxDot-dev-mbed5 commit history version, 5.4.7 at this time.