You're out of space, so you have to use a custom firmware with something removed. If you can figure out how to get a serial log you shouldn't have much problems building an image using the guides.
https://openwrt.org/docs/guide-user/additional-software/beginners-build-guide
https://openwrt.org/docs/guide-user/additional-software/saving-space
There is also a web based builder that you can use with the instructions from ginn13 about what to remove: Online ImageBuilder and Upgrade Server
The drawback is that it's harder to know if anything malicious has been added if you use the online builder.