Transmit power is limited to 1 milliwatt by law. Power can be reduced but not increased above that limit. The default setting is full power. It is best to locate the router as high as possible in a centrally located position. That said, 7 year old equipment is most likely using 802.11g which is very slow and 3 generations old. Here are typical specs for each wifi version.
Code:
802.11 SPECIFICATIONS
Max Indoor Channel
Bands Speed Range Width
(GHz) (Mbps) (ft) (MHz)
11ac 5 1300 230 20/40/80/160
11n 2.4, 5 450 230 20/40
11a 5 54 95 20
11g 2.4 54 170 20
11b 2.4 11 150
Max Speed for 802.11 n & ac varies depending on the
number of antennas. Speed for 3 antennas is shown.
802.11 a b & g use only 1 antenna.
Range will vary depending on obstructions.
There are some things you can do to with hardware improve your network.
Connect the router to the repeater with an Ethernet cable. The throughput will increase significantly.
Upgrade the wifi adapter where possible. There are USB wifi "n" adapters for less than $10.
If your router supports a guest network, you can split into an 802.11n only network and an 802.11 b & g network only.
That way you'll have a network for the slower devices.
To reduce the effects of interference use only 20 MHz wide channels on the 2.4 GHz band (ch 1 - 11).
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