Hi all. I am helping my elderly father-in-law who needs to be able to use his iPhone outside on his lakefront property. This is a picture of it below. There is no cell coverage, so the only way to make this happen is wifi calling.  In the end, I need to get a strong reliable signal in the shaded area on the left, without requiring that he switch network names.  For ease of use reasons, whether he is in the house, or the garage, he has to just be able to dial a number without needing to choose a network.

The property has a main house with 100mbps internet using the ISP supplied router (see orange dot). 

I am going to run ethernet cable to the toolshed from the router inside the house.

My goal is to get the strongest possible signal on the property where he may be roaming far from the house (see shaded blue area on left). 

This is where I need help:

1. For this goal, would the best option be to connect one EAP225 at the toolshed, another on the garage, perhaps a third elsewhere to blanket coverage and then set them up as an Omada mesh network?
2. If so, since there is no mesh network in the house (the ISP provided router is sufficient for now), to achieve the single network name mentioned above, what device would I need to place in the house.

Thanks in advance!