I used to mock them, then I tried them, now I recommend them in situations like this. TP-Link powerline ethernet adapters (homeplugs) will sort your problem out nicely - A pair can be had for about £25 from anywhere, even ASDA!
I stuck the TL-PA4010KIT in at my mum's house, one located near to the router and the other by their smart TV.
If you're short of sockets, the slightly more expensive TL-PA4020PKIT will mean you don't effectively lose two sockets. Try to avoid running them off extension cables if possible.
The AV standard for homeplugs means they're broadly interchangeable between brands, and you can buy and add additional adapters to the network as you wish, just pairing them as you go. (The TP-Link packs come pre-paired.)
I'd also buy a small Netgear Prosafe gigabit 8-port switch for your upstairs setup (GS108UK - £23 from BroadbandBuyer - https://www.broadbandbuyer.com/products/824-netgear-gs108uk/) or TP-Link's knockoff, the SG108 (https://www.amazon.co.uk/TP-LINK-TL-SG108-8-Ports-Ethernet-Wall-Mounting/dp/B00A121WN6) for a few quid less.
I use a Netgear GS108 and it's run flawlessly for years, except for a couple of times when it's needed a power cycle. Not bad for about ten years 24/7 operation.
This is preferable to running a second router, as if you do that you'll likely be doing a dual NAT (or repeating wireless networks, itself problematic). Running multiple routers causes all sorts of port forwarding and PAT issues down the line. Plus, if your wireless signal is weak upstairs already, you'll get compromised speeds even if you manage to get a wireless repeater working - just no point. I've tried it and given up on it for those reasons.
Homeplugs operate on the physical layer (transposing RF on the power cabling) and present to the devices at either end like a run of cable. They work just about everywhere with no noticeable loss of speed, except if you have the world's oldest house wiring.
You can also buy powerline adapters which have a built-in wifi AP/repeater - but these use the wired connection to feed the their wireless AP, which is immediately better than trying to receive a weak or interference-prone signal from downstairs. You might want to add one of those at a later date if you've got wireless devices upstairs.
