Advice on how to put Hub 5 into Modem mode & what router to buy

Good choices above, but dont jump in just yet - see this as a starter to get some ideas.
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The cognoscenti will always say (correctly) that the best thing to to do is connect Wireless Access Points directly to the Hub or your own better router by ethernet cables. However some would say a good Mesh will be the best/simplest/most flexible for most average users who dont want to run ethernet cables. .  A Tri-band Mesh system is better for the "backhaul" and for users on high speed packages and umpteen simultaneous connections. Wifi6 is future proofing for getting speeds over 600 mbps on wifi.

Another option is to extend the ethernet connections (and wifi) via powerline adaptors - they work well for lots of people - including for my "gaming son" in his bedroom and they can be "cheap and cheerful".

It depends on your requirements for... better routing features, your house layout, size, construction materials, your tech abilities, and of course, budget, ~£40-80 will get a decent introductory wireless router (TP-Link Archer C6) but 'may' not on its own solve the wifi in your house, but spending ~£500 will get the “Rolls Royce” solution of a high-end Wifi6 Router & WAP's or a top end Tri-band Mesh system with WiFi6 (may cost a bit more). You'll probably want something suitable - somewhere within that range - I reckon ~£100-150 is around the “sweet spot” for most standard users, and where you would probably need to start for a standard 2/3-story house if it has brick walls. Less for a one-bedroom modern build flat, more for a stone built 8 bedroom mansion

Good options for Access points/Wifi routers/Mesh systems come from Ubiquiti, Asus, TP-Link Deco's M4/M5, Linksys Velop's, Netgear Orbi's (and others) – I would stick with these companies whose business this is - rather than “newcomer” and data-driven” companies like Google, Amazon, etc. Also be sure the router or Mesh specifically says that they have 1GB ethernet ports - some of the cheaper ones only have 100Mbps ports. WiFi6 is also future proofing for faster connections.

Recently, just as an example... three of my friends who all live in brick built 3/4 bed semis, just got TP-Link Deco M5 3-packs (~£150) and all said it is - “absolutely brilliant with excellent wifi speeds in all rooms, the attic and the garden, and the App is a doddle to use”. The first Mesh unit becomes the new router - you need to check whether the one you look at has all the "features” you need. Ethernet ports are usually limited to 1 or 2/unit , so you “may” need to add in an unmanaged GB port switch (<£20) in there as well. My mates also get "full speeds" from the ethernet ports on the Deco's in the remote locations. Finally if you get a 2/3-pack of any Mesh system, which doesn’t cover dead spots everywhere, you can just add in extra unit(s).

Take a look at what fits your needs, tech ability, house layout and budget. Use sites like Techradar & Techadvisor for reviews of "Best Routers" or "Best Mesh". Post again if you want personal recommendations from others on your short list