GSM Interference with Meanwell Switch mode PSU
You need to first measure if the PSU is indeed the problem. I would suggest looking at the spectrum on the output to see what frequency it is switching at, and to verify that this could be an issue. Even then, it is hard to really 'prove' as you don't know what IF frequencies are used in your modem.

If it is indeed the cause, through what means is it doing so: Radiated? Through the input power supply? On the output? 
Only then can you start working on a remedy - worst case, get a different model of PSU that has different switching frequencies and/or better EMI performance.