3.0.0.4.378.4850, just stock firmware from ASUS. Bandwidths are 20/40MHZ with channels set to auto. Armstrong Cable, up to 30MB/s. Placement I now isn't great, 3 story townhouse, it's on the bottom level, but I feel like even when there the speed isn't great. Probably not 3 feet from the wall behind, but there's nothing pulling from that direction.You can start with more details.
What firmware are you running and what are your ISP speeds?
What options other than defaults have you set? What channels are you using? Have you tested each channel to find the one with the highest throughput and lowest latency?
(Btw, forget about using wifi utilities like inssider to choose the best channel, they are useless; simply test each channel and see what works best in your environment).
How far and what obstacles are between the router and the clients?
Where is the router located physically? I would recommend at least 10 feet above ground level with no obstructions like walls, shelves or inside closets or bookshelves for at least 2 or 3 feet in each direction and located as centrally as possible in 3D space of the most used wifi areas of the building it is installed in.
2.4GHz and 5GHz bands are slow vs. anything newer than an RT-N66U or higher (but the RT-N56U with padavan firmware is still great for a bridge though).
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