Since AD623 was out of stock, i ended up buying something like this: https://www.arduino-tech.com/cjmcu-8221-ad8221ar-precision-instrumentation-amplifier-module/.
My concern is now: how should i or better said, where should i connect the GND pin from the module?
As far as i can tell, after analyzing the breakout board, the module comes equipped with the input resistances, gain resistance and the capacitors outlined by me in the red square. I followed the presented scheme accordingly, built the driven ground circuit and connected the +VSS pin to the V+ output, the -VSS pin to the V- output and the GND pin to the AGND. Afterwards, since i don't want diode protection for the time being and since the previous assumption, that the module already has the input resistances, i took the 47K ones and put them on the left of the 1M ones with the view of acting as protection resistors. Everything else was kept intact, i only disconnected the right driven leg circuit, because i also don't need it from now, but i was unable to obtain any output.
I should mention that i tried to isolate only the INA block to see if it works, and this is ,i think, where the problem is located. The datasheet (https://www.analog.com/media/en/technical-documentation/data-sheets/AD8221.pdf) specifies to tie REF to the appropriate local ground, so i assumed it must be linked with the AGND. So this is my configuration at the time being: -VSS pin to V-; GND pin to AGND; +VSS pin to V+; REF pin to AGND. On the IN+/IN- i tried to gather signal both from electrodes located on the skin and from a 5khz generator but i got nothing. The generator was connected with its ground to AGND, and with its output to one 47K resistor and then on both 47k resistors tied together and no result. The only concerning result i had was the fact that at one point i was reading from the output a 50Hz signal, whereas both my circuit and the generator where powered from 9V batteries, and the oscilloscope, a Hantek 6022 one, was connected to the PC, which was not plugged into the power-line socket.
So, again, what i am asking is: How to properly connect the breakout board, since there must be a problem with the AGND driven ground potential and the mismatch with the REF/GND pins from the module. (I will also recheck my soldering job for any discontinuity, just to be sure.)
Thank you for your answers!