The problem comes from the cable...It is too long !!

The built-in antenna gain of TL-WA5210G is 12dBi. Considering the loss on the connector and your 34m cable, I dont think using a 15dbi omni antenna can have a good performance as the built-in one in long-distance transmission.
Even with the same gain value, a omni antenna needs several times the input power to achieve the same distance of a directional antenna.

But you can try your luck by some adjustment in the webUI.

1. go to the Management Page -> Wireless -> Wireless Advanced -> Antenna Setting, choose "External Antenna" instead.
2. set the highest transmit-power.
3. Wireless->Distance settings should be correct.