Unfortunately, it is merely an exercise in elimination, of identifying the weakest link in your communications channel.
Although its an annoyance to relocate equipment, move the server into the same room for a day, and test it at different periods to see if the problem continues.
If not, the problem is defintely wireless droppage.
If it does continue then plug the server into the router with a cable. If it still drops connection, then it would point to being a broadband issue, not a wi-fi issue.
Type broadband speed checker into google and run one of those speed tests, and check the spped against what the ISP is supposedly giving you. This might indicate a bad broadband connection.
The two most likely options are::
1. The router itself is of low quality, and cannot cope with transmitting packets , hence the make and model fo the router would point to its usefulness.
2. You said server was two rooms away. Are they stud walls or solid, and are there other obstacles in the line of sight. I had occasion once where two wireless cards had good signal, through one stud wall and one solid wall, yet intermittently dropped signal. It was finally tracked down to when the secretary was standing at the photocopier.