Why do you prefer 5GHz over 2.4GHz wifi network?
Unless you have a cordless telephones around I suggest opting for the 2.4GHz frequency.
I quote " The higher the frequency of a wireless signal, the shorter its range. Thus, 2.4 GHz networks cover a substantially larger range than 5 GHz wireless networks. In particular, the higher frequency wireless signals of 5 GHz networks do not penetrate solid objects nearly as well as do 2.4 GHz signals, limiting their reach inside homes."
and quote "Some people mistakenly believe 5 GHz network technology is newer or somehow more innovative than 2.4 GHz."
Some things to consider,
Can you try another USB port for your dongle?
Are you using a USB Hub? (Remove it)
Do you have many USB devices attached at the same time? (usually this is limiting USB bandwidth)
Do you have any other devices besides a phone that may interfere with the signal? (refrigerator, microwave etc)
How many dbs is your signal strength?
Model: DS1512+, DX510 FW: DSM 4.0-2228 HDD: 5 * 1.5TB, 5*1TB.
Other: HP 1080G-8GE Switch (LACP + Jumbo Frames), Lancom Router + PBX 1723B,
Linksys POE switch, TimeCapsule 2TB, Baudtek Router, Pireli IPTV, Dune-HD D1