The Modbus gateway feature supports only pass through of the following function codes to the RTU slaves:
01, 02, 03, 04, 05, 06, 15, 16, 22, 23.
Other unsupported function codes (you mentioned 2B hex?) should not be used with the gateway.
Note that the gateway actually also supports remote programming of other TRI PLCs that are connected to the gateway via RS485/232 as slave. However, only one type of protocol (modbus or hostlink command) may be used at a time on the same comm port.