I managed to solve this. The firmware seemed to be US version (and the newest at Tplink support site). This was very confusing because I am living in Finland and has a second-hand device. I managed to flash ddwrt (US) firmware (but for some reason, no wifi) and then I made a factory revert. After that I managed to update to the newest EU firmware. Actually I liked the ddwrt so much (except for lack of wifi, I think this was because of US firmware) that I then upgraded again to ddwrt (non-US). After that everything is working very well.

@Oober Ditto, same problem, same solution.

In US (WA state) got 2x C7 v2 and both are (CA). Maybe TP-Link distributed CA version here in US? Maybe WA is near CA and I happen to get 2 CA imports? Don't know why. Once CA version makes into US and previous owner did an upgrade, they likely chose US firmware which is how US firmware got into this router?

Starting with TP-Link's 180114 firmware, tried to flash to international version of DDWRT (since CA version) and got the 18005 error. Followed Oober's process and flashed 6 times to finally got to international DDWRT. The sequence was

- US factory-to-ddwrt

- US ddwrt for C7 v2 (I picked r36330)

- ArcherC7v2_webrevert ( https://forum.dd-wrt.com/phpBB2/viewtopic.php?p=936939#936939 )

- newest international ArcherC7v2 firmware

- INT factory-to-ddwrt

- INT ddwrt for C7 v2 (I picked r36330)

I did all this wirelessly so had to hop between 192.168.0.1 (TP-Link) and 192.168.1.1 (dd-wrt). Might be easier using ethernet to finish the whole job haha.