
If Chimera fails repeatedly (rare, but possible), use checkra1n. This uses a hardware-level bootrom exploit (checkm8), which is un-patchable by Apple. However, it requires a computer every time you reboot.
Download the Checkra1n ISO and use a tool like BalenaEtcher to flash it onto a USB thumb drive. How To Jailbreak Ipad Air Ios 12.5.7-