It is difficult to see where they are soldering in the attached pictures as they show up small for me. They also seem to highlight two different soldering points.
The instructions at
https://apollo.open-resource.org/lab:argus#modifications just solder at the AN 2 connector. Cutting the board is necessary if you follow those instructions.
An unmodified MR3020 should reach about 100m in an open field. I tried to find the range of one with an external antenna and couldn't find any info. If the modification hasn't introduced any problems, a standard 5dB antenna should extend the range--especially indoors. Remember, extending the range too far can lead to problems where the device you are using to connect doesn't have enough power to transmit back to the MR3020.
Unless you want to portability of the MR3020 you may want to look at a router like the TL-WR842ND. It has two external antennas and can be found for quite cheap.
Regards,
Don