So, I tried another methods that were provided on archer c6/a6 v3 installation site, first I had to connect to serial port and when I tried to install openwrt via the recovery method, it didn't went through and this was printed in console:
Firmware verify OK!
firmware(size 0x694d0e) verify pass!
HTTP upload is done! Upgrading...
do http upgrade
nm_tpFirmwareRecovery---
Firmware Recovery file length : 6901006
Firmware process common.
Image verify OK!
Firmware file Verify ok!
[NM_Debug](nm_buildUpgradeStruct) 01273: nm_buildUpgradeStruct 1273 87fe6d54
[NM_Debug](nm_buildUpgradeStruct) 01291: nFileBytes = 6896890
[NM_Debug](nm_lib_readPtnFromNvram) 00569: partition_used_len = 267, requried len = 4096
[Error]sysmgr_proinfo_buildStruct(): 660 @ unknown id(device_name), skip it.
[Error]sysmgr_proinfo_buildStruct(): 660 @ unknown id(country), skip it.
[Error]sysmgr_cfg_checkSupportList(): 1043 @ productVer 3.0.0 NOT Match.
[Error]sysmgr_cfg_checkSupportList(): 911 @ Entry 0 NOT Match.
[Error]sysmgr_cfg_checkSupportList(): 1043 @ productVer 3.0.0 NOT Match.
[Error]sysmgr_cfg_checkSupportList(): 911 @ Entry 1 NOT Match.
[Error]sysmgr_cfg_checkSupportList(): 1043 @ productVer 3.0.0 NOT Match.
[Error]sysmgr_cfg_checkSupportList(): 911 @ Entry 2 NOT Match.
[Error]sysmgr_cfg_checkSupportList(): 1043 @ productVer 3.0.0 NOT Match.
[Error]sysmgr_cfg_checkSupportList(): 911 @ Entry 3 NOT Match.
Firmware not supports, check failed.
[NM_Error](nm_checkUpdateContent) 01211: the firmware is not for this model
[NM_Error](nm_buildUpgradeStruct) 01316: checkUpdateContent failed.
Web recovery failed type 0.
## Error: HTTP ugrade failed!
so it seems like it's a problem with version of my router (because I've got the v3.2 one) but maybe someone with knowledge on that will look into it. Fortunately for me, method of OEM installing with TFTP worked for me. Hope the easy way of installing will be fixed soon