Can not save any files and changes

I built my own firmware for raspberry. But I can not save any changes (new folders, files, configs) after rebooting.

ouput of mount; df -h:

root@miromico:~# mount; df -h
/dev/root on /rom type squashfs (ro,relatime)
proc on /proc type proc (rw,nosuid,nodev,noexec,noatime)
sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,noatime)
cgroup2 on /sys/fs/cgroup type cgroup2 (rw,nosuid,nodev,noexec,relatime,nsdelegate)
tmpfs on /tmp type tmpfs (rw,nosuid,nodev,noatime)
tmpfs on /tmp/root type tmpfs (rw,noatime,mode=755)
overlayfs:/tmp/root on / type overlay (rw,noatime,lowerdir=/,upperdir=/tmp/root/upper,workdir=/tmp/root/work)
tmpfs on /dev type tmpfs (rw,nosuid,relatime,size=512k,mode=755)
devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,mode=600,ptmxmode=000)
debugfs on /sys/kernel/debug type debugfs (rw,noatime)
none on /sys/fs/bpf type bpf (rw,nosuid,nodev,noexec,noatime,mode=700)
Filesystem                Size      Used Available Use% Mounted on
/dev/root                12.3M     12.3M         0 100% /rom
tmpfs                     1.9G     72.0K      1.9G   0% /tmp
tmpfs                     1.9G    120.0K      1.9G   0% /tmp/root
overlayfs:/tmp/root       1.9G    120.0K      1.9G   0% /
tmpfs                   512.0K         0    512.0K   0% /dev

version is OpenWrt 21.02.1, r16325-88151b8303

root@miromico:~# cat /etc/banner
  _______                     ________        __
 |       |.-----.-----.-----.|  |  |  |.----.|  |_
 |   -   ||  _  |  -__|     ||  |  |  ||   _||   _|
 |_______||   __|_____|__|__||________||__|  |____|
          |__| W I R E L E S S   F R E E D O M
 OpenWrt 21.02.1, r16325-88151b8303

Please, help, how to fix it?