Technically you can replace Ethernet with WIFI - but it is a very bad idea if your business systems rely on Internet - even moreso with your choice of equipment.
From a computers point if view, once the WIFI is working it works much the same way as an Ethernet connection (In fact, in the vast, vast majority of cases where you have both Ethernet and WIFI on a device it will prefer Ethernet when available and fall back to WIFI seemlessly, but won't use wifi to supplement Ethernet)
The problem is that you have very little control of the environment with WIFI, and, especially in a busy area, you will get decreases in speed depending on what anyone else - not only your business - is doing. Ethernet does not suffer from this problem. (I've seen businesses not able to work because of WIFI congestion of others - using better equipment then you have)
Also, the more people you have using the connection, the worse it will run (6 people using it at the same time will slow things down by more then 6 times)
This is made worse because your router does not support the 5gig band / the current standard (802.11ac) which means it icant take advantage of the techniques to reduce interference and take advantage of the more appropriate higher frequencies. - In fact your bottleneck is no longer your ISP connection, it will be your WIFI.
As others have pointed out, WIFI is not secure, and it is very possible for someone to break in to your network without being detectable.
You really should fix your Ethernet infrastructure instead of relying on WIFI.