It's a router that's managed by the building, I'm afraid, so I can't access any settings on it, only infer them. I'll edit in the resolv.conf.

It seems you're using DHCP. 1) When you use the phone hotspot, you get access. 2) When you use the building Wi-Fi, you don't get access and your resolv.conf shows 127.0.0.1. Is this correct? If yes, the problem's in the building's systems - you're not getting valid IP configuration via DHCP.

I couldn't once connect to internet because of wrong MTU size

@PBS that's it then. The building's router must provide you a valid IP address and DNS address. 127.0.0.1 is your own system - it can direct you nowhere unless you actually run a DNS server, and even then only if there's an internet access to begin with so your DNS can communicate with public DNS servers.

NetworkManager can and will overwrite any manual edits in resolv.conf.

@Cbhihe: No, I'm not using tor. I believe the loopback address is because I'm using dnsmasq. Also, how does pinging 8.8.8.8 involve dns?

@Cbhihe: Set resolv.conf to include 8.8.4.4. The edit survives, but it makes no difference.

@Cbhihe: This isn't an Arch Linux issue. As I said, it also happens on Debian. The linked thread is about a gross misconfiguration, whereas my config has worked up to now.

Watch sudo journalctl --follow (or sudo tail -f /var/log/syslog) as you try to connect to the router.

Reread man NetworkManager.conf. Your dhcp=internal is what's preventing your system from getting a DHCP-assigned IP address from the router. No IP address, no TCP/IP communication.

@waltinator: I've added the system log upon connecting. It looks normal enough to me. As for dhcp=internal, it seems like my only option as the others are dhclient and dhcpd.

did you compare logs - 1) connecting via phone->mobile data, 2) no internet via phone-> router?

@Andra: No, but you would expect them to be be very different even if both were working.

@Cbhihe: Just switched to dhclient. I still can't ping anything, but bizarrely I now have internet access. Wtf?! Does this router not support ping?

Solved. Verdict: Some crappy switch further along the line than my router doesn't support ping. Also, NetworkManager's internal dhcp client is too basic to be able to converse with this weird, crappy switch. So the workaround is to use the more featureful dhclient.

Don't know how you can infer that the block of the ping facility does not occur at router level, but further along. This is not the main post issue, but a lot of "managed" routers disable ICMP incoming and outgoing and you should not base probing your access to the Internet on whether ping returns anything at all.