As others asked what is the intention or goal for doing this? At a high level if the intent is to provide a measure of back up, fail over, resiliency. All three depending upon the primary goal requires planning, finances, and hardware.
As it relates to power vs data there are limitations and trade offs to accomplish the same.
A simple example of power redundancy / fail over is where you connect the camera to 12 VDC. Almost all (POE Enabled) IP cameras today that detect 12 VDC will automatically disconnect the POE portion within the camera.
Thus only data comes off the Ethernet cable to the network infrastructure.
Should 12 VDC be lost the Ethernet line can be energized to provide both data & power. Keep in mind depending upon hardware in place some need to be rebooted to re-enable POE. In this simple example power back up / fail over can be achieved but NOT data connectivity.
To achieve data back up / fail over requires a lot more planning, trade offs, and investment of resources / finances.
Again, based on the example stated up above lets assume both 12 VDC & POE is in place.
How than do we achieve a measure of data back up / fail over???
Again, if the camera is powered via 12 VDC and incorporates edge recording (Micro SD) inside. It doesn't matter if the Ethernet cable is disconnected or compromised as the camera will continue to operate and record data on board. This allows 24.7.365 video security, data retention, and backfill (ANR), once the Ethernet / POE Switch is issues are resolved.
Another method to achieve fail over / redundancy is to incorporate a mini POE passthrough switch. Thus you have two parallel lines connected and available to each target camera system. It goes without saying a lot of planning, validation, automation, and follow up is required to make such a system operate correctly without impacting the network.
The costs to deploy the same becomes very expensive . . .
Other solutions are to purchase or add on equipment that allow a target camera to send its same data via RF. It's safe to say there aren't very many if any mission critical systems that run on RF alone vs used as a method of fail over / back up as its intent.
The same problem is seen and arises when a dedicated WiFi enabled camera is in place. Some hardware allow both WiFi & Ethernet to be seen on the network but data only flows out one or the other. Whereas some allow both at the same time but Ethernet has priority over WiFi.
Then, there are cameras that simply won't operate at all if the hardline (Ethernet) is connected while the RF (WiFi) portion is enabled.
Hence there is a lot of trial and error, proof of concept, and endless testing and validation to assure what is seen is consistent in operation and behavior!
So, as noted you need to define a clear goal and understand the pros / cons of each solution.
NOTE: You'll probably notice I have not touched upon the phrase Resiliency As this adds a whole new level of planning, resources, and finances . . . You may Google 2N / N+1 as it relates to networking, power distribution.
Again as previously stated by myself and others, since we don't know what it is you're wanting to achieve, it's difficult to assist you or provide the best answer.
FWIW, if redundancy / fail-over operation during a power outage is a consideration, then install an appropriately sized UPS on the POE NVR and/or POE switch.
Also, many cams can be configured to send an email alert if there is loss of signal, etc.