I can't find much info about the HG556a. But I have found the user guide
https://broadband.vodafone.ie/download/files/HuaweiHG556aServiceManual.pdf
Can't see any reference to 802.11n - unless this is a different router to yours that just happens to be named the same. Who's the manufacturer? The manual is for one made by Huawei
This is the closest I can find to a spec - only mentions 802.11b & g:
Huawei HG556a 300M 3G wireless router:
International brands Vodafone (Vodafone) customized products company
Built ADSL2 (for ADSL broadband, plug devices directly to the telephone line), 3 USB interface (supports USB2.0 standard), support for 3G wireless router. That is, ordinary ADSL lines and dual-use equipment, 3G routers, CPU uses Broadcom BCM6358, 300MHZ processor, FLASH for the 16M, 64M large memory.
Item Specification:
An external fixed omni-directional antenna
The transmission distance: Furthest indoor 100 meters, outdoor 300 meters far
Support: IEEE 802.11g, IEEE 802.11b, IEEE 802.3, IEEE 802.3u, IEEE 802.3x, CSMA / CA, CSMA / CD, TCP / IP, DHCP, ICMP, NAT, PPPoE
LAN port: 4 10/100M adaptive RJ45 Port
WAN port: 1 10/100M adaptive RJ45 Port
Frequency Range: 2.4 ~ 2.4835GHz
11g: 54/48/36/24/18/12/9/6M
11b: 11/5.5/2/1M
Data modulation: BPSK, QPSK, CCK and OFDM (BPSK/QPSK/16-QAM / 64-QAM
RF Power: 17dBm
External power adapter output: 9V ~ 50Hz 0.8A
Package Including:
Huawei HG556a X 1
Power adapter X 1
ADSL Micro-Filter X 1
Telephone Cable X 1
PC/LAN Cable X 1
Base X 1