I have a problem with applications like Whatsapp or Signal which are installed on PC - is this only 1 PC, why not PC have static IP address from DHCP, if this is only requirement for the PC and add as suggested the rule for that PC not all the source IP address.
capture the logs and see any uniqueness you can see in the header, so based on that you can make a rule, for example like user-agent.
Another option explicit configuration on the browser, which is not suggested but if you like to test,