How many SMs are registered to this AP? What kind of bandwidth are they configured for?
What does the link test with bridging and MIR return. I know you said not the same, but better or worse. I'm going to assume it was better (what was expected). Is this the same SM you started the thread with? Is it different, but on the same AP? If the link test with MIR returns the correct results it will verify the RF link is ok. If none of the SMs on that AP can achieve the correct bandwidth through ethernet, make sure that the AP can do so. How many APs share the backhaul? How are they performing? Connect to the switch at the tower and run a speedtest to the server from there.
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Take a look at this, may help.
Post your link test results. I didn't mention enabling SNR calculation on SM link test, but do so.