Thanks for the quick response. I understand that 5HG is faster and travel shorter distance while 2.4 G is slower but travels longer. What I am trying to find out id if what I observed is normal (i.e. is there any problem with my equipment or set up). I believe , my ISP is providing what they promise because I am getting 73.4/10 with my 5G connection. I am getting exactly 75/10 using ethernet connection. I believe an average 802.11n 2.4G adapter should get at least 100 mbs. Why I am only getting 33.4 Mb when my phone is connected to my 2.4G SSID ? To further isolate the issue, I used my laptop in place of my phone, the result is similar 34 Mb down with 2.4G and 74 Mb down with 5G. So this means it is not my phone's 2.4G adapter. This may mean that the problem is the 2.4G of the CGN3 modem/router. I set up an wired access point using an old linksys E3200 by connecting the E3200 to CGN3 using ethernet LAN and set up wireless SSIDs (E3200_2.4 and E3200_5). All these set ups are within 5 or 6 feet distance (i.e. no signals travelling through any walls). The result is similar. 70 Mb down when connect to E3200_5 and 30 Mb down when connect to E3200_2.4. With the AP set up, I am using the 2.4G radio of the E3200 instead of the CNG3. This seems to says that both the 2.4G radio of the E3200 2.4G radio are similar (could be both bad or both OK).
In summary, I need to understand why I am only getting about half of what my ISP provide on 2.4G ?