Zyxel LTE5398-M904 connected to the cellular network but no Internet access

would you mind to copy and paste here the text you wrote when you opened your ticket ? I think it's most effective if we all phrase it the same, so they cannot think we are talking different issues.
Of course remove any sensible data in case you agree :)

After a few months, here is my feedback…

There are, at least, two issues:

First:

• When using Illiad provider (named "Free" in France) default APN is IPv4. Every 12 or 24 hours DHCP is renewed and the Zyxel seems to get lost. LTE is ok, but Internet cannot establish new TCP connection due to this IP address issue. Moreover, if a TCP tunnel is previously existing (OpenVPN for example) it keeps working showing that the IP stack is still alive… which is interesting because it means that NAT is able to use the new address on this tunnel. I suspect the new IP adress is not correctly handled at a certain level of the stack but not all… strange. Could it be a firewall issue that blocks the new address, which could explain why an existing tunnel works? My 2 cents.
• You can manually force IPv6 with Illiad on your mobile account, than change APN configuration to IPv4/IPv6. In this case, the issue disapears and the Zyxel handles correctly the changing IPv4 address for 2-5 days or more. However, here comes the 2nd issue :-)

Second:

• I have 2 providers (Free by Illiad and Sosh by Orange), and both BTS are away from home. I can get RSRP of -92 and SINR 4/6 on bands BC20 and BC28, but -102 of less and SINR -2/0 on bands BC1, BC3 and BC7. For unknow reasons, Zyxel loves to go with BC1 or BC3: speed is pretty good for some hours, but suddenly the connection gets lost. It can be after 5 minutes, 5 hours or 5 days… Depending on the weather, it can be after a few seconds: when I restart the Zyxel it connects to BC20 or BC28, and after a few seconds switches to BC3 and breaks (both Sosh and Free).
• I managed to fix this by manually disabling bands: only BC20 and BC28 are left. Obviously the speed is lower as aggregation is not working as expected anymore, but at least the signal is sufficient to keep the connection for weeks.

Of course 1st problem is a firmware issue on the IPv4 stack, or probably on the internal firewall. 2nd problem is probably on the BTS side, I suspect the signal being too poor and the providers cut off the link, but the Zyxel doesn't detect it. Anyway, not an IP issue but something at the LTE layer.

Anyway, a watchdog would fix this. I've installed my script on my pfSense firewall, and it handles things right since a month or so, with forced BC20/BC28 (as resetting LTE every few minutes due to poor BC1/BC3 doesn't help to get a stable connection!).

Regards.

@JeanFrancois Hey how do I check if my router is currently using IPv4 or IPv6?
In which tab is this information located?

@fabriba

It should be rather "Cellular APN", but I found this option :)

You can set IPv4/IPv6 there, but I don't know which option the router is currently using, whether IPv4 or IPv6.

It is only shown that it uses IPv4v6.

I went into System Info -> WAN Information and found the following information.
Does the "N/A" in the "IPv6 Address" line mean that the router is using IPv4?

There are some addresses in the black rectangles and I covered them.

I don't know about it :(