My router is an asus rt-ac68u and I have tomato installed on it (I wonder if that's the problem..). The wifi adapter is netgear a6100. The testing was performed in an internal network. I will keep troubleshooting.
Stock firmware is known to have the best wireless. If you want a more featureful alternative firmware, try AsusWRT-merlin. It is practically an enhanced stock.
I tried merlin before and I'm really torn.. tomato does some things clearly better (dns-crypt, ftps, file sharing, bit torrent, UI, customization) and I wouldn't give up tomato if it wasn't strictly necessary.
