You could run iperf3 on the router as server and test with the PC as client to check your LAN connectiviy.
Also check the CPU load with htop on the router.
# opkg update && opkg install iperf3 htop
Start the server.
# iperf3 -s -V -D
Start htop to monitor CPU load when you speedtest your Internet or your LAN:
# htop
Start htop as client on the PC
iperf3.exe -c [IP address of your router]
These are the results from my PC with a Linksys WRT1200AC.
iperf3.exe -c 192.168.90.254
Connecting to host 192.168.90.254, port 5201
[ 4] local 192.168.90.220 port 53523 connected to 192.168.90.254 port 5201
[ ID] Interval Transfer Bandwidth
[ 4] 0.00-1.00 sec 112 MBytes 936 Mbits/sec
[ 4] 1.00-2.00 sec 111 MBytes 933 Mbits/sec
[ 4] 2.00-3.00 sec 112 MBytes 933 Mbits/sec
[ 4] 3.00-4.00 sec 111 MBytes 936 Mbits/sec
[ 4] 4.00-5.00 sec 111 MBytes 935 Mbits/sec
[ 4] 5.00-6.00 sec 111 MBytes 934 Mbits/sec
[ 4] 6.00-7.00 sec 111 MBytes 929 Mbits/sec
[ 4] 7.00-8.00 sec 111 MBytes 935 Mbits/sec
[ 4] 8.00-9.00 sec 111 MBytes 935 Mbits/sec
[ 4] 9.00-10.00 sec 111 MBytes 929 Mbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bandwidth
[ 4] 0.00-10.00 sec 1.09 GBytes 933 Mbits/sec sender
[ 4] 0.00-10.00 sec 1.09 GBytes 933 Mbits/sec receiver