You have a couple of choices..
- A wifi extender that can connect to your weak signal and boost it.
- A wifi router that you can cable into your current network and provide a new wifi access point (old bt hubs are good for this)
- Power line adapters with wifi, if you have a decent power circuit in the house.
- Use your phone as a hotspot to grab the occasional reading from the ispindel.
- An ispindel nautilis relay to display the ispindel data and boost the output to your home WiFi.and send data to brewfather etc.
What you chose depends on what you want to achieve. The general wifi improvement methods will benefit other network access, but you need to plumb into your home network somehow. The nautilis is great for the ispindel, supports two, adds an extra temp sensor, gives a cool display and boosts the WiFi output (you can even fit an antenna extension to it)... But it's a specific solution for the ispindel.
(I sell the ispindels and nautilis on ebay as gingerneil. But if you fancy giving one a go then drop me a message and I can do them for £35 - cheaper than ebay as I'll save some fees).