Please check the UART Flow control eFuse configuration on ATBTLC1000-ZR module. Refer to section Hardware Flow Control for 4-Wire Mode eFuse Write Procedure, ATBTLC1000 BluSDK Example Profiles Application User's Guide (
http://ww1.microchip.com/downloads/en/DeviceDoc/50002640A.pdf).
If the ATBTLC1000-ZR module eFuse is not configured for "4-wire UART" mode, you can configure the UART Flow control to 4-wire mode (refer to the document pointed above).