I know running new wires can be a pain. When sharing the wifi signal, internet is going to be shared as well. However the cameras, will not use internet data as long as you don't plan on sending the feed to a cloud server. It will go into the NVR.
I would suggest that you move the router closer to the cameras and use the wireless range extender for sharing internet in the rest of the house. Position the router so that there are least number of walls between it and the cameras. If the camera feed is unstable, turn on motion detection, lowering the frame rate or resolution will also reduce data rate coming from the cameras. Most current cameras support H264 compression which is an older standard. H265/HEVC is the new video compression standard. At the moment there are very few cameras that support H265. The advantage of H265 is 50% reduction in video file size as compared with H264. As far as choice of range extender is concerned, I would suggest you read some reviews before you select one. Performance can vary widely across brands.
The other option is powerline ethernet its much faster than a range extender. It uses two powerline adapters. One is connected to the main router and then plugged into a socket. The other is plugged into another electrical outlet where you want the wifi signal extended. The data transfer happens through the electrical wiring in your home. So theoretically, you can plug the second powerline adapter into any electrical outlet and get a Wifi signal anywhere in the house. The downside is they cannot work through a UPS since it filters out the data signal. Without a UPS, if the power goes down, you lose the connection.
