Try adding

--no-check-certificate

It may be part of the letsencrypt CA expiry.

They route the internet fine

What about DNS?

For example, you can ping 8.8.8.8, but can you ping google.com?

Yes, I've forgot to mention this! When using the pinging in the Diagnostics tab, I'm getting this error when trying to ping openwrt.org:

ping: bad address 'openwrt.org

or

PING 8.8.8.8 (8.8.8.8): 56 data bytes
ping: sendto: Network unreachable

or

ping: bad address 'google.com'

I am using the IPv4 pinging from there

On the mi4 Lan interface, make sure the gateway address of the lede router is populated. It would make sense that clients are able to route because they get gateway info from the lede dhcp , but the mi4 is probably staticly set.

Oh, good catch! I missed that dhcp wasn't enabled.

Gateway in the Mi router (openwrt) is set to my LEDE router, however to no avail :/
tyvm for the tip tho!

If anyone else runs into this problem.

I had to disable WAN6 interface, since for some reason my Modem/router was not routing IPv6 from inside (between OpenWRT and Modem) to external.

Had the same problem, disable IPV6 in every device in network > interface > devices. Restart the router.

Late to the party but what actually fixed it for me was changing the timezone and matching the dates within the router. nothing else I did fixed it