While I was out toiling in the front yard, I was thinking about your predicament.
While it's hard to come up with a solution without seeing your setup and the layout of your home, I suspect what's happening is that your extender is too far away from your router. You said that room had a dead spot, which means the wifi signal from the router isn't making it to that room or that part of the room.
So if you placed your extender anywhere close to that dead spot, guess what it's extending? The very weak signal that does make it there from the router. So the solution is to find a happy medium.
Run those speed tests. If your regular speed from Spectrum at the router is say, 100Mbps, then wherever your extender is now, you may be only getting like 1Mbps. Remember, what you bought is an extender, and it can only extend what it's getting. If it's seeing a weak signal, it's extending that weak signal. It's not a signal booster, only an extender of what it receives. make sense? Move the extender 10 feet closer to the router, if possible, keeping a relatively clear line of sight, if possible. Then go back in that room and run another speed test and keep moving the extender until you have decent enough speeds in that room.
Another thing to remember is, the ISPs don't hand out the best equipment to start with so your router may not be sending out a nice strong wifi signal throughout your home and to your extender. Spectrum charges you $5 per month for the router they provide and unless times have changed, they are notorious for giving customers the cheapest modems and routers on the market so you may want to go out and get your own router like a Netgear Nighthawk or one of the TP-Link Archer models, over time it'll pay for itself with the $5 a month off your bill and better speeds. Of course you'd have to know how to set it up on your network, but there are tons of videos and tutorials out there if you ever decide to go that route.
Hope that helps and if you have any questions, fire away