You have 14days no questions asked return on Apple purchases from Apple store..
I would definitely open the plastic.. not all plastics are transparent to microwaves.. it could be making it worse.
Your experience could then be that the TC does not improve the wifi at all.. this would in fact be totally expected in 2.4ghz band.. and with older computers..
The improvement will only be at 5ghz and generally later equipment.. or AC wireless.
Are you joining 5ghz.. ?? If you don't know force the connection to 5ghz by using a different name.
Your setup is fine.. do not use guest network . . it is slow.. maybe by design.
And you cannot use guest in bridge mode.. that is normal.
Turn off the wireless in the ISP modem so you can measure properly how the TC is going.
Fix the wireless channels and use 11, 6, 1 for 2.4ghz in that order.
And use around 40 and around 150 for 5ghz.. the lower channel is often much better.
Do not expect great improvement with old clients.. it just ain't gonna happen.
Just return it.. no sweat.
The Asus RT-AC66U (or even N66U) are the power house for domestic WiFI.
Even the new TP-Link AC model is up there.. but Apple's is well down the list..
Although this comparison is using PC client.
@LaPastenague - your fix worked a treat!! I have recently installed a new Airport and whilst Wifi reception on my Macbook Pro was fine - other non Apple devices (Windows laptop/android tablets and phone) really struggled and kept disconnecting. My steps exactly to get improved connectivity were:
Choose WPA/WPA2 Personal as security settings of the Airport insteand of just WPA2 Personal
I then set the 2.5ghz channel to 6 and the 5ghz to 40.
Wifi has been performing perfectly across all devices since....