I have a TP-Link Archer c60 v3 running openwrt. I would like to revert to the latest stock firmware (https://static.tp-link.com/2021/202101/20210118/Archer%20C60(EU)_V3_201231.zip) but there is a problem. The software version of the latest firmware is 1.2.0 while it should be 3.2.0 as the router expects a soft_ver of at least 3.0.0 (the firmware that originally came with the router). This was also a problem with the first stock firmware (https://static.tp-link.com/2019/201902/20190219/Archer%20C60(EU)_V3.0_190115.zip) but someone managed to change the soft_ver and re-calculate the firmware checksum in the header (Help in unbricking TP-Link Archer C60 V3 - #21 by adrianschmutzler) I know how to change the soft_ver with a hex editor but I don't understand how the checksum at the start of the header is calculated and how to fix it. Could someone please help me?
Update: I tried using mktplinkfw and mktplinkfw2 from the openwrt build system but both complained that there wasn't a v1, v2 or v3 header. I also tried using tplink-safeloader but although it seemed to be compatible with the header I didn't find any option for recalculating or even showing the checksum.