Your question is confusing because most people talk about wifi inside their house. Are using some form of wireless to receive your internet from the ISP.
The systems used by ISP to provide internet via wireless have many different systems that are not actually "wifi". The best known is stuff like 4G LTE used by cell phone companies. But many WISP also use stuff like LTE and many other proprietary non "wifi" methods to deliver service. Some very cheap providers will try to use actual wifi but it has all kinds of limitation which is why all quality providers use other methods.
There are too many variable to say why you only get the speeds you do. It could be the signal level is too low. Using a different router may not help that unless it also say works on a different radio channel that you get stronger signals. If this was wifi in your house and you were using 802.11n at 150 you would generally see about 30-40mbps.
Well that's quite insightful as you mentioned, As far as I know my entire setup is capable of allot more Mbps than what the ISP claims it can sustainably handle.
On the task manager in windows, on my usb wifi dongle 300Mbps, wirelessly connected to my indoor Wifi Router 300Mbps, wired connected to my outdoor Wireless Router 150Mbps, connected to the access point a few clicks away on the mountain I have had eyes on speeds on this very same setup up to 50Mbps and upwards for short periods of time eg: an entire weekend none stop.
Why my ISP would claim that higher speeds are not going to work or their exact words not going to "sync" with your quite old equipment is beyond understanding other than they feel that I am using my connection to much already as it is, and that they can only make money if they sell new equipment for an use with higher speeds packages they have on offer that currently will not work with my equipment. (4Mbps down/ 1Mbps up) even thou their service provides package speeds of 100Mbps and higher with 13ms or less latency.
I have even had my eyes on the outdoor router admin page a few times and saw that the signal quality and strength is is a very respectable 90% plus.
So as it stands now I can only download 20Gb per day on my current hardware at this speed, witch cant be upgraded unless new equipment is obtained.