Although many routers provided by the ISP are not the best it is not as bad as it has been in the past. For things like basic wireless they provide acceptable performance. Most difference are for some of the advanced wifi features that are only useful if you end devices also support them.
A range extender/repeater as you suspect will not fix this problem. These devices need a good signal to start with and then they repeat it which then degrades the signal by repeating it. If you start with a bad signal it only gets worse not better.
You could try to use a router as a AP and just turn off the radios in the current isp router. That would in effect be replacing just the wifi part of the ISP router. I have my doubts if it will...