Unable to get Quectel EC25-E to connect in QMI mode

Installing and Using OpenWrt Network and Wireless Configuration
21

Here is the working "IPv6-only" configuration, tested with EE pay-as-you-go SIM:

config interface 'wan'
        option proto 'qmi'
        option device '/dev/cdc-wdm0'
        option apn 'everywhere'
        option auth 'none'
        option ipv6 '1'
        option pdptype 'ipv6'

config interface 'clat'
        option proto '464xlat'

Interface clat was added manually (yes, QMI interface will not start it automatically) and put into wan firewall zone. There is no need to configure anything else, the default configuration just works.

System log:

Thu Feb 16 10:09:58 2023 kern.info kernel: [  121.802989] nat46: adding device (464-clat)
Thu Feb 16 10:09:58 2023 kern.warn kernel: [  121.803415] [nat46] make_nat46_instance: allocated nat46 instance with 1 pairs
Thu Feb 16 10:09:58 2023 kern.warn kernel: [  121.815906] nat46: netdevice nat46 '464-clat' created successfully.
Thu Feb 16 10:09:58 2023 kern.info kernel: [  121.816264] nat46: configure device (464-clat) with 'local.style NONE local.v4 192.0.0.1/32 local.v6 2a01:4c8:x:x:x:x:x:x/128 remote.style RFC6052 remote.v6 64:ff9b::c000:aa/96'
Thu Feb 16 10:09:58 2023 daemon.notice netifd: Interface 'clat' is now up
Thu Feb 16 10:09:58 2023 daemon.notice netifd: Network device '464-clat' link is up
Thu Feb 16 10:09:58 2023 user.notice firewall: Reloading firewall due to ifup of clat (464-clat)

Now testing with IPv4-only web site:

root@MF286D:~# wget -s http://ipv4.tlund.se
Downloading 'http://ipv4.tlund.se'
Connecting to 64:ff9b::c10f:e4c3:80
(null)                8172  --:--:-- ETA
Download completed (8172 bytes)

The IP address shown in the output is automatically provided by DNS64 server on ISP side.
64:ff9b:: is a prefix and c10f:e4c3 is a representation of the IPv4 address of the web server, 193.15.228.195
To be completely accurate, up to now this is how DNS64 works, not 464XLAT.
464XLAT comes into play when we need to reach "literal IPv4" address, when DNS is not involved.

Trying to reach the same server by its IP address:

root@MF286D:~# ping 193.15.228.195
PING 193.15.228.195 (193.15.228.195): 56 data bytes
64 bytes from 193.15.228.195: seq=0 ttl=48 time=96.105 ms
64 bytes from 193.15.228.195: seq=1 ttl=48 time=55.325 ms
64 bytes from 193.15.228.195: seq=2 ttl=48 time=55.993 ms

root@MF286D:~# traceroute 193.15.228.195
traceroute to 193.15.228.195 (193.15.228.195), 30 hops max, 46 byte packets
 1  192.0.0.1 (192.0.0.1)  0.048 ms  *  0.091 ms
 2  192.0.0.1 (192.0.0.1)  0.082 ms  0.086 ms  0.087 ms

Some explanation about IPv6-only operation on EE network is available here.

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