In Modbus, a slave is silent unless spoken to by the master. A slave can not start/initiate a Modbus query/message/poll.
If the HMI is used for issuing commands, like start or stop, then the HMI must be the master and the PLC must be the slave.
Field instruments or analyzers are almost always Modbus slaves. To accomplish Modbus RTU communications between a PLC and Modbus field instruments, then you need either
a PLC that can operate as a master on one comm port to communicate with the field instruments, and as a Modbus slave on a 2nd comm port to communicate to the HMI, or
a transfer data transfer scheme where the master HMI gets data from the slave field instruments and then writes that data to the slave PLC using separate transactions.