Boost signal (Hardwarely) - no configuration

I take it you're not using a *pair* of the Loco M5's, right?  
The best way, if at all possible, is with an antenna.  In your case, something like a parabolic dish aimed at the AP.
POE will do nothing but bring power to something that already likely needs power - and the device needs to be ready for that.

Also, it should be line of sight.  No trees, etc. interfering with the path.  Again, that's an antenna kind of thing in that the idea is to get energy from A to B.

It doesn't look like the Ubiquiti Loco M5 has an option for adding an antenna cable.  Maybe inside the case?  If not then there are others like the Engenius access points that have external antenna connectors.

but the signal is poor because Loco doesnt have enough power!!

No!

The power at your end has absolutely nothing to do with received signal. Fred's right. What you need is either...

1) More power from the far end, or
2) a bigger ear at your end. (ie, dish or other parabolic antenna)

Well, it is a two-way thing so adequate signal at each receiver does matter at both ends.
But *power* is a poor way to get that when it's already almost working or even, well, anyway.  So Craig Beck is right about that.

A dish at one end will help in both directions because it will improve the receive signal to noise ratio and it will narrow the beam (as in "focus") the output power.

Having a dish at one end but not at the other may do it, but it's not guaranteed.

Increasing power at your end would not improve anything though if you can barely hear the far end. TX power at your end does NOT improve received signal at your end. Gain and TX power are two separate things.

My assumption has been that you have no control over the far end, it being a "public access point".
So, everything needs to be done at your end, right?
It's always true that less improvement is less of a guarantee than more improvement.  
As I tried to say before, a dish at your end will do these things:

1) Improve the signal to noise ratio of the signal you receive by rejecting noise from sources all around.  This is equivalent (in a signal to noise sense) to the source increasing its power but is a much more efficient way to get signal gain.  It's also equivalent to putting a dish at the source that's pointed at *you* to the exclusion of others who are at any reasonable distance away from the source.  Of course, neither of the latter two are possible for you.

2) Improve *your* transmitted signal level at the far end by "focusing" your transmitted energy in that direction.  This is equivalent (in a signal to noise sense) to increasing your transmit power but is a much more efficient way to get signal gain. And, it comes for free if you really need #1 above.  I agree with Craig Beck that this may be the least of your needs and that increasing your transmit power instead isn't advisable.

So while gain and transmit power are indeed two different physical things, their affects add up in the signal level equations together.