I'm glad to see this answer. I'm in the same situation - my ISP sent me a modem/router. I use that. Don't need much else. It's even got a guest network (not that we've had many untrustworthy visitors in the past couple years).
But I'm also glad to see the other answers here because we recently moved the home office to another room slightly further away and we're going to need to throw some WiFi boosters/repeaters in here.
Same. I found that my neighbours were swamping my signal in the outer rooms of my (small) flat and did the booster thing, a couple of ethernet-over-power wifi routers: TP-Link TL-WPA8631Ps
They were a breeze to set up, pushbutton if you just want to use the same SSID/password as your ISP router. I have this set up to serve the 5GHz on a different SSID tho - that stops my laptop switching to 2.4 and terrible performance. I'd have done this on the ISP's router (a BT homehub) and just pushed the buttons, but since a recent firmware update, the homehub goes into a crashloop if you try this.
If you have coax wired through the house, I strongly recommend MoCa instead of powerline Ethernet. It works very well and doesn’t turn your house into a shortwave radio noise generator.
If it's any help, I used to use Devolo EoP devices, but I'm stopping. My 1930s house has crummy wiring and I was frequently having to power cycle them to get anything to work.
I've switched to using a TP-Link Deco M5 wi-fi mesh (and I've disabled the wi-fi on the router my internet provider supplied).
So far, no problems at all. They're much cheaper also.
My local ISP’s hardware shares my bandwidth with others - something I won’t tolerate. God forbid egress addresses are shared and I get accused of something.
