Hi Cris89, I am a bit unclear on what exactly your setup is, are you using the HG612 as a modem with a router - which router is it?
If your connection is good then one simple way to improve wifi is to do what are suggesting, no need to change your modem / router and just add a WAP, like the Unifi one you suggest.
If I was buying a new one I might be tempted to get one of the new wifi 6 standard ones for future proofing. Personally I use the Ubiquiti AP AC Pro's and they have been excellent but there are lots of choices, the Ubiquiti ones are not necessarily the most easy to set up, in my experience, due to Ubiquiti's proprietary software - while being quite good is also an additional layer of complication that some people don't like, they are also quite expensive.
Good luck
C