Router’s Guest Network Won’t Isolate Devices Like It Should, Even After Firmware Update

I recently updated the firmware on my ASUS RT-AX58U router to the latest version hoping to fix some connectivity quirks. One big reason was to get proper guest network isolation working - I want guests to access the internet but not see my main devices. However, after the update, devices on the guest network still seem to ping or access shared folders on my main network, which is a security no-no for me. I’ve double-checked the guest network settings to ensure isolation is enabled and even reset the router to factory defaults before reconfiguring. I also tried toggling AP isolation and tested with multiple devices (phones, a laptop). The router’s admin page shows guest isolation as active, but in practice, devices can still 'see' each other across networks. Has anyone else had problems with guest network isolation not working properly on ASUS routers after a firmware update? Is there a hidden setting or workaround to truly lock down guest access? Would switching to a third-party firmware like Merlin be worth the hassle for this? I want to keep the main network secure without buying new hardware if possible.