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Signal drop when using BLE module on custom PCB

Hello!
I have been developing a BLE application on nRF52DK and used the ACN52832 as a module for a custom board for experimenting with BLE. I ordered a custom PCB from Oshpark and soldered the ACN module to it. If anyone can recommend a similar prototyping service to Oshpark.com, based in Europe, I would appreciate. I would like to upload the gerbers, get a price and pay - easy.
Unfortunately, I have discovered a problem with signal strength. When the ACN52832 is soldered to the PCB, then the RSSI drops around -20dbm. This makes the PCB unusable.
When the ACN module is standing separate from the PCB, just powered on, then it has a much better RSSI.
What have I done incorrectly on my PCB design? Or could the PCB material affect the radio? It seems that if I would have left the ACN's PCB antenna outside of the PCB dimensions then the signal strength would be better...If you can point me in the right direction with a link/source to explanation then I would appreciate it. This is the first time I encountered something like this in my hobby.
Attached you can find two images that represent the situation. I covered some silkscreen for privacy.
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