[Solved] Tplink Archer C7: problem with 2.4GHz wifi, it caps at 20Mbit

Client used is Dell Latitude E7440

Since i have two routers (v2 and v5), output below is for both. Behavior/speed is the same on both.

v2:

root@archerc7v2:~# iw dev wlan1 station dump
Station 20:3d:bd:9f:78:5a (on wlan1)
	inactive time:	920 ms
	rx bytes:	3720
	rx packets:	19
	tx bytes:	489
	tx packets:	5
	tx retries:	0
	tx failed:	0
	rx drop misc:	0
	signal:  	-44 [-52, -45, -59] dBm
	signal avg:	-44 [-52, -44, -59] dBm
	tx bitrate:	1.0 MBit/s
	rx bitrate:	72.2 MBit/s MCS 7 short GI
	authorized:	yes
	authenticated:	yes
	associated:	yes
	preamble:	short
	WMM/WME:	no
	MFP:		no
	TDLS peer:	no
	DTIM period:	2
	beacon interval:100
	short preamble:	yes
	short slot time:yes
	connected time:	9 seconds

v5:

root@archerc7v5:~# iw dev wlan1 station dump
Station d8:fc:93:32:d4:cb (on wlan1)
	inactive time:	0 ms
	rx bytes:	10181695
	rx packets:	65060
	tx bytes:	110178720
	tx packets:	95627
	tx retries:	4104
	tx failed:	64
	rx drop misc:	57
	signal:  	-53 [-66, -55, -58] dBm
	signal avg:	-53 [-65, -54, -59] dBm
	tx bitrate:	54.0 MBit/s
	rx bitrate:	54.0 MBit/s
	expected throughput:	37.994Mbps
	authorized:	yes
	authenticated:	yes
	associated:	yes
	preamble:	short
	WMM/WME:	no
	MFP:		no
	TDLS peer:	no
	DTIM period:	2
	beacon interval:100
	short preamble:	yes
	short slot time:yes
	connected time:	3618 seconds
Station cc:61:e5:9a:fc:5e (on wlan1)
	inactive time:	3820 ms
	rx bytes:	1343530
	rx packets:	7504
	tx bytes:	13133455
	tx packets:	10268
	tx retries:	616
	tx failed:	1
	rx drop misc:	0
	signal:  	-25 [-27, -33, -36] dBm
	signal avg:	-27 [-29, -34, -38] dBm
	tx bitrate:	54.0 MBit/s
	rx bitrate:	6.0 MBit/s
	expected throughput:	37.994Mbps
	authorized:	yes
	authenticated:	yes
	associated:	yes
	preamble:	short
	WMM/WME:	no
	MFP:		no
	TDLS peer:	no
	DTIM period:	2
	beacon interval:100
	short preamble:	yes
	short slot time:yes
	connected time:	3515 seconds