What is the POE power source injector, switch, NVR?!? If the camera is truly going off line every 3-4 days and remains off I would confirm that state.
Off to me is literally it’s not powered vs not accessible which can be how you access the device. Case in point many users will complain they can’t view the feed over a smart application on their cell / tablet.
The vast majority of times this has been the software / hardware was too bloated to the hardware was underpowered to display more than X video feeds.
When the camera is in this state my first question and task would be to ping the device and validate its even on the network. Pinging a device and receiving an Ack doesn’t mean anything besides at a network (hardware) level that it (NIC) works.
I would than try to login to the camera locally, via app, PC Software, NVR if present. If all show no connection that eliminates a cell / tablet smart application issue and hardware. Because you could still access the same via PC / NVR.
If that also shows no access you need to determine if it’s a network issue vs power. Power should be easily simply by powering the same with a separate POE injector, 12 VDC, another POE Switch.
If the camera never goes off line that narrows the problem to a lack of power. It doesn’t address if you have bad RJ45 connectors or cabling which should have been verified first at install and during your trouble shooting now.
Split the trouble shooting into sections which you can do now and move on to others that are more involved etc.