WIFI now is indispensable and here are some tips if you have problems with your WIFI signal in your retail shop.
Measure the problem
You cannot do much till you have measurements, the WIFI app that I like to use to help me judge WIFI signal strength is WIFI Overview 360 Pro available on google play free. I suggest you first get a copy.
Now look at the position of the modem
All too often technicians tend to put the cables so modems in the office area which is an out of the way place in the shop. As walls and doors all block the WIFI signal so putting the modem in the office room is often wrong. This is not good for WIFI modems. The WIFI modem should be as central as possible in the shop hopefully near the main areas where people use WIFI.
Move things away
Move away from anything from the modem that blocks the signal.
What we find works well is to raise the WIFI modem, the higher the better. Because it sometimes is an eyesore what you can do is put the modem in a nice material bag and hang it up on the wall. Do not use a plastic bag for this.
A neat trick here which you can experiment with to boost the WIFI signal is put silver foil between the modem and the wall. Some WIFI signal in the shop will bounce off the metal foil back onto the modem.
Reduce the number of devices you use
Try reducing the number of items using the WIFI in the shop, too many WIFI devices can cause problems. Maybe you cannot provide all your staff with WIFI in the shop to their mobile phone.
Reduce electrical interference
Try to reduce the electrical interference particularly from other WIFI modems. I remember going to a neighbours shop and talking to the owner there to make some compromise in the positions of the WIFI modems as their modem was interfering with our client's modem. This is a particularly big problem in shopping centres where there are heaps of WIFI modems, cordless phones, etc all belching radio signals.
Try changing the channels on your modem
Try changing the channels that your WIFI modem uses. As a rule, 5 GHz is faster but works on a shorter distance while 2.4 GHz gives a longer range. Experiment with both and see what works best for you.
Try putting the antennas in different directions say one vertically and the other flat. WIFI signals come from all different directions.
So there it is, now see how these tips go for you.