I'm using two SSIDs at home for my network: one for 5GHz and one fore 2.4GHz. Both networks share the same IP range with the same connection settings: gateway and my own PiHole as DNS server.

This sunday, out of the blue, being connected to the 5GHz network I noticed I couldn't connect to anything on the internet. I disabled/enabled the wifi, nothing changes. I restarted the phone, nothing changed. I checked all my devices at home (personal laptop, work laptop, Android TV device - nVidia Shield) and they all work on the 5GHz network.

Then, I connected to the 2.4GHz and it works without any issue at all.

I then wanted to see if my phone could resolve DNS. I have termux installed and running a nslookup command I could see it uses Google's 8.8.8.8 DNS server, which my DHCP is not sending. This is frustrating to say at least. Anyway, it's the same DNS server when connected to 5GHz or 2.4GHz, but for 5 it doesn't even reach the server for some reason.

I also set the phone to "forget" the wifi network and then reconnected, with no luck.

Also, it's very frustrating that Google hides network settings. I can't see the DNS servers configured for my connection anywhere. I had to install an application to be able to see them.

Any idea how to get over this?

Is the issue only present on the phone? If you were to clone the phones Mac address on a laptop or desktop and connect it do you get the same results?

As far as I can tell, yes, only on the phone. Also, the phone is configured (by default) to randomize de MAC address, so I don't think I can replicate it. I'll try to check on the router if it connects with different MACs every time.

Try to disable IPv6 on your router settings, and then try connecting to 5GHz band wifi from your Oneplus smartphone. Xiaomi HyperOS also has a similar issue.

I have termux installed and running a  nslookup  command I could see it uses Google's 8.8.8.8 DNS server, which my DHCP is not sending.

By default Termux uses DNS form resolv.conf, not the DNS that android system uses.

Yes, I found out today. Stupid android....