Ethernet cable connections are the gold standard - wifi a poor second. It will work very well. Is it just for temporary use or will it be "semi-permanent"? Will you still want the Hub to be used downstairs for wifi/ethernet connectivity too? If so just leave everything as it is on the Hub and run Cat5e or Cat6 cabling up to the box room. 20-50 meter runs are fairly cheap and you can get "flat" cable to tuck behind carpets etc.
Then just plug a laptop/computer into the cable. If you want multiple ethernet connections in the box room (e.g. consoles?) - just add a £15 unmanaged Network switch with 5-8 additional 1GB ports and plug into them.
However - if you want better wifi too then a good introductory WAP would be the one below. You would set it up as per the manual instructions to be in Access Point Mode or "Bridge mode". It has x4 GB ports on the back too which will give the best connections
But the advantage of this one is that it is also a "Router" so you could also use it as a wireless router with the Hub in modem mode. It will be a far better router (more controls - see the specs) than that in the Hub and its wifi will also be far superior to the Hub's. Just play around and select what works best for you layout
Amazon is a good place to shop for kit because of quick delivery and excellent returns policy.
https://www.amazon.co.uk/Archer-C6-Wireless-Supports-Parental/dp/B07L5YRYF4/
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John
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I do not work for VM. My services: HD TV on VIP (+ Sky Sports & Movies & BT sport), x3 V6 boxes (1 wired 2 WiFi,) SH2 in modem mode with Airport Extreme Router +2 Airport Express's. On VIVID200, Talk Anytime Phone, x2 Mobile SIM only iPhones.