I would suggest calling into Rogers and speaking to their technical support for wireless. But before you do that do a sim change and see if it will eliminate the sim card from being defective. Also i wanted to add are you in a house or building?
The signal bars just give a very rough idea as to signal strength. More useful readings should be available by reading signal strength in your IPhone settings. I don't have an IPhone so can't help there. However, on my Android phone I use a program "Network Signal Info" which shows signal quality without having to go into the phone settings. It also shows the location of the tower to which I connect. No doubt similar utilities are available for the IPhone.
It should make it easier to determine your signal quality under different conditions. How is the signal outside the house? Knowing that can be useful for knowing the best signal you can expect at your location. Also, are there higher buildings or trees blocking the signal? You may be able to determine that by trying your phone at different locations in your neighbourhood.
To illustrate; I am 6 km from the nearest cell tower but it is behind a huge hill so I cannot get a connection. Instead, my phone connects to a tower 30 km away. If I walk 1/2 km to where the hill does not block the closer tower too much(although I still cannot see it) it connects to the closer tower with a stronger signal.
It looks like you may be able to change frequencies by going to Settings-->Cellular (or Mobile). It looks like it depends on the provider.
It may be premature to assume it is a Rogers problem. Is your IPhone 6S plus the same phone you were using with Wind mobile? There can be quite a difference in phones' capabilities to receive signals, even within the same manufacturer's models. For example, I have read several articles complaining about poor performance with Samsung's newest model S7 compared to their older models.
Seeing degraded performance in all areas where you had better performance before may suggest it has something to do with the phone's characteristics(assuming you were using a different phone with Wind Mobile). Hopefully IPhone users will see your post and provide knowledgable suggestions.
Here's an interesting article to show how to turn show an IPhone's signal strength in numbers rather than bars: