It all depends on a) how fast you want your network to be and b) how much configuration you want for it.
For a simple solution, simply add one new access point - either centrally in your property (APs designed for one purpose tend to offer much better coverage than ISP routers that even when advertised as ‘best wifi ever’ are built to a cost and make comprimises) or simply extend ethernet to the weaker areas and place the AP there.
Something like this would do the job fine in this regard:
If, however you want a more advanced network, you may be better off turning off the wifi aspect of your ISPs router and going with either the TP Link or Unifi units that are designed for commcerial use. TP Links are a bit cheaper but a bit slower - with that said it depends on your actual line speed. If you are on a 300mb or gigabit fibre connection, you’ll want the wifi to be as fast as possible. If on the other hand you are getting anywhere between 5-100 meg, a 300meg access point will be more than enough.
I have found two that may be suitable and linked them below. I would go for the Unifi if you need the higher speed, if you don’t the TP Link is a good way to save some money - perhaps buy two TP Links for even better coverage, if needed. You can always buy one and see how well it performs before adding another later on if required.
With both of these they can be manged through a server running on your network that allows you to record performance stats, usage etc. However this is optional, and both the TP Link and Unifi units only need to connect to the server software once in order to be configured - once you’ve got them set up how you like, they will run on their own quite happily with no further input.
Both of the above can be powered by PoE - this means you only need to run an ethernet cable to the access point and it will get its power through this - handy if you’re limited by where your plug sockets are.