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

did you try what I suggested above? network settings > broadband > cellular wan (modify) 

on my modem I can choose between ipv4 and ipv4/v6, I don't think yours would be different. I am on V1.00(ABQV.3)C0

@BruCom

In general, last week I changed the operator to T-mobile to check if the router will not drop the connection.

Internet speed increased from 131 Mbps to 179 Mbps when aggregating the two bands BC_1 and BC_3.
I changed the location of the router, which turned it off and on again.

On May 28 at 14:30 my Internet connection broke (the LEDs on the router were green, but in the router panel there was an error of no connection in the "Connectivity" window, after refreshing the page it disappeared) :D
I did a reboot and the connection came back.

On May 30, around 23:30, I changed the setting to IPv4v6 and recorded the logs.
I have reviewed them now and do I understand correctly that the router lost its connection to the LTE network, but managed to restore it in a few seconds and changed the IPv4 address?
The BTS station number is still the same, but I covered it.

Do my logs look similar to yours?

So far, I have no problems with the Internet until the reboot.

The longest time without problems is 5 days and a few hours.

By the way, I saw you @BruCom got an A11 update file from Quectel. Is it different from the one on the NetGear website?

@JeanFrancois In "Celluar APN" I set IPv4v6 and since 11 days I have no problems with disconnecting the Internet.
I checked if the IPv4 address changes and yes it does router loses connection and reconnects after a few seconds.

It is true that I changed the network from Orange to T-Mobile, but it also disconnected me from the internet before changing to IPv4v6.
I am using V3 router software version and A11 LTE model software

Today is the last day to return the router and stay with me :)

Thank you very much for the tip to change the setting to IPv4v6 :D

Seen, thanx. I'll give it a try next week when I'll be back home: I've also encountered the aggregation limitation, and if v11 can improve the behaviour it can be a good point.