at the RS485 side repeater may work. both sides on one repeater can be up to 50m concerning the ProfibusDB standard.
I don't know how the HMI is communicating to the PLC. If it use ASCII, then some of the repeater with auto-detect-baudrate may not work. This kind of repeater requires a ProfibusDB sign to detect the right baudrate. If ASCII is used between HMI and PLC then the ProfibusDB sign will not be created and so the repeater can not detect the baud rate.
Other option is to use a Siemens Panel because by default ProfibusDB is used for the communication and up to 50m is default. No ASCII is used. So it will work without any additional HW.
Hope this helps