you need to solder the 2x8 pinheader on the upside of the PoE board like you can see in this picture:

Now solder the 1x6 pinheader on the underside:

Start with the modem, take care of the orientation and take your time.

Like you can see in the picture it is first not very nicely done. Remove the solder which is too much with a desoldering braid.

The board can take all two versions of the LCD (switched power pins). Just solder the correct pads together. Take the two pictures as an example. Don't solder the LCD now on the PCB.

Solder the 2x8 pinheader on the downside of the PCB. I am using a tweezers to get the board straight for simpler solder joints.

Add the two buttons on the PCB and solder them.

Now solder the LCD and you are finished with the modem board.

Stack the two boards together with the spacers and the nuts.

Congratulations the complete build is finished now.