Strange 3 Router issue

I've been using a TP-Link MR6400 4G router with 3 for more than 2 years now with no problems. I was finding moving large files around my wired network rather slow. This is when I noticed that this router was only a 10/100 model and was only CAT4 as well so I decided to try an upgrade.

I bought a new 3 branded CAT12/Gigabit ZTE MF286D from eBay and have been very happy with it for a couple of weeks. It's increased file transfer speed by the expected factor of 10 and doubled my download speed from 40mbs to over 80.

Last night while watching iPlayer, video ground to a halt. Since then I've found I can only reach certain internet sites.
I can reach:
Google
YouTube
Googlemail
The Guardian
Amazon
Facebook
Thinkbroadband

I can't reach:
BBC News/Weather/iPlayer
eBay
Ookla Speedtest
Twitter
ExpressVPN
Tidal
and many more

Can anyone suggest what is going on here? My limited knowledge makes me think it's a DNS issue, but I can't seem to change it in the locked down 3 firmware in the router.

Try the old router. At the moment we don't know if the router is the problem or the network

Should have said, I've gone back to the TP-Link which seems to be working normally.

Can you access the inaccessible pages by IP address?
google.com has address 142.250.200.14

I can access google directly. I probably should try with address of one I can't?

In reply to a post by cookie:

I can access google directly. I probably should try with address of one I can't?

Of course. My bad.

As far as I can see, the sites on the first list are all reachable via IPv6 (as well as IPv4 of course), whereas those on the second list are IPv4-only - at least, the ones I know.

Therefore this could be a problem with IPv4 transit at Three, or with NAT - either on your router or somewhere in Three's network. If rebooting your router didn't fix it, then it's something deeper than your router's NAT getting stuck.

Are you sure you don't have something connected into your own network which might be acting as a DHCP server? Like another router? That could redirect all your IPv4 traffic elsewhere, but leave IPv6 working fine.

If this problem happens again, it's worth checking your own IPv4 address and your gateway IPv4 address ("ipconfig /all" on Windows), and checking that the gateway IPv4 address you see is the wireless router's LAN address. Also try:

tracert -n 8.8.8.8

to test outbound connectivity on IPv4.

There is another router on the network - an old AirPort Extreme. I wondered if this was an issue so have tried with it disconnected with the same result. I've even just plugged the 3 router directly into my main. Mac not connected elsewhere.

I have only Macs so I'll have to look up the relevant info for finding IP addresses.

I dashed the first post off a bit quickly and I was keeping it simple. I probably should have mentioned that when the problem first occurred and stopped me watching IPlayer, I switched wi-fi off on my iPhone and checked how that was. Got the same result as the router - limited connectivity. This was still the same next day until I rebooted the phone which fixed the issue. Re booting the router does nothing to help.

Thanks all for the help BTW

Plugged the router again and it's been working normally for the last couple of days.