The product pages for Atlas Scientific EZO pH & EC circuits have datasheet pdfs that explain how to switch between UART and I2C mode:
I’m not familiar with the switch @sllo mentioned in that comment.
In the picture from the next comment, there’s one big momentary switch with a red button next to the BNC jack. That one is labeled SW1 RESET, so presumably it’s for resetting the Arduino. It looks like there might be a couple tiny switches with white buttons near C5. I can’t read their labels due to the glare from the LEDs, but it’s a good sign that there’s two of them (possibly one for each EZO module).
The procedure from the datasheets for switching to I2C mode involves disconnecting the EZO module’s GND, TX, and RX from the host circuit, shorting a couple pins on the EZO module, then applying power to the module until the LED changes color.
You could try following the traces for the things near C5 that look like tiny switches with white buttons. Is one wired to short TX to the upper PRB on the EC module? Is the other wired to short TX to PGND on the pH module? If so, that’s a good sign.
If that doesn’t make sense, perhaps you could post a good picture of your sensor board that shows how the components are populated.
[edit: Assuming the small white buttons short the right pins, if you tried pushing them while the EZO boards were already powered up, that wouldn’t be following the instructions from the datasheets. If you unplug the sensor board from the Arduino and hold the buttons down while cycling the power to the sensor board, that would come a lot closer to following the instructions. The datasheet makes it sound like the EZO module firmware might only check for the shorted pin when the board first powers up.]