I'm currently using TP-Link TL-WDR3600 v1, which works very well for me. However, I'm looking to extend range a little bit. Other than adding second device, a range extender or something, is there any decent openwrt router that would have better range so I can replace my router with it (ideally white in colour )?
I'm not really interested in increasing power, but increasing sensitivity possible. Whatever gives better range.
From my initial investigation it looks like device with mt7621/mt7628 might be the way to go?
very tempting! do you happen to know if there are different versions A and B as with BT hub? Or do I just look into buying any "Plusnet Hub One"?
To the best of my knowledge, the Plusnet Hub One only exists in a single configuration, equivalent to the supported Type A. Be aware that the initial flashing method is not for the faint of heart, but it's rewarded by a pretty nice and capable device for very little money, but read up on https://openwrt.ebilan.co.uk/viewtopic.php?f=7&t=266 first. If you don't feel comfortable about doing this, several sellers offer these devices with OpenWrt preflashed.
In terms of range, don't expect any router to be much better than another, but the BT HH 5a is surprisingly good in this regard. If you want a significant improvement, get two and set up https://openwrt.org/docs/guide-user/network/wifi/atheroswds.
Disclaimer: I do own a tl-wdr3600 and a BT Business Hub 5 Type A and have compared them against each other, the BT HH 5a is pretty good in that regard (the 802.11ac 5 GHz wlan making most of the difference).
 I have noticed that on average, 802.11ac manages to get the throughput better over the range than 802.11n (as in your tl-wdr3600), but it doesn't really improve the absolute range. 802.11ax provides another speedup relative to 802.11ac, but again the absolute range doesn't change that much.