The FFmpeg package was used for collect the stream from the IP camera and store that in files by piecemeal (2 minutes length of each part). OpenWrt OS haves FFmpeg build in, but this version not work with H264/265 stream at this moment, despite this stream type is most used in cheap high resolution IP-cameras.
But up on OpenWrt OS (lightweight Linux) can be installed Debian OS (fullweight Linux). Then fullweight FFmpeg package can be installed at Debian and this version can works with H264/265 stream from IP cameras.
First step was dramatically increased the memory of the router with a USB flash (but USB-HDD or USB-SSD is best). Same time, this storage can be used for video archive if no NAS in the home network. The Samba was installed.
Second step was installed fullweight Debian OS under debootstrap.
Third step the FFmpeg was installed.
The IP camera can be connected to the router wired or Wi-Fi.
Using ONVIF Device Manager was found the link to the camera's RTSP stream.
With noted RTSP link of the camera, the Crontab of OpenWrt was configured for save video from IP camera in files and for control of the video archive size.
That's all. Ever since this router is not only router, but video recorder too. Was no experiment with two ore more IP cameras, but test with one only IP camera do not show overload of the router's CPU.
The video archive can be shared not only in the home network, but with global access in Internet too. For make this, need to install FTP server in OpenWrt, and use router's port mapping with static IP, external IP, or temporary IP with alerts about them changes.
If the router's computer planned to use only as video recorder, then is good idea to switch off Wi-Fi radios on a board - will be less EM radiation and more power for USB.
See "Diary" here for detailed instructions and commands case.