No upload on 4G

Hi everyone. My name is Igor, without a hump 😀. I'm a long lost computer technician, now working as an electrician. I have a strange problem and I'm hoping that someone will be able to help me with it.

I bought a router, Asus 4G AC68U, and it's a decent router, with a few bugs that are circumventable, but there is one that has been bugging me a lot, for a long, long time.

I'm using a 4G LTE sim card as a WAN, to access internet. The problem is when I use 4G, three devices have a problem, they can download but there is no upload, but when I switch to 3G everything is great, except the terrible speed of 7mbps, so 3G is just for resolving the upload issue, when absolutely needed. The 2 devices are mobile phones, Blackview BV9800Pro, Ulefone Power Armour 13 and the third is a Android box used as IPTV, all bought in China. The Xiaomi phone bought in my country works both 3G and 4G, of course al of this is through WiFi. Now, I know this sounds as an obvious thing, but there is a catch!

If I take the SIM from a router and plug it in any of these phones, 4G has speed over 100mbps download and over 30mbps upload???

I even tried to make a hotspot on Xaomi, when router is in 4G, Xiaomi has both download and upload, but Ulefone and Blackview have no upload?

It's a horrible problem I have not been able to resolve. Can anyone please try to forward me to a possible solution?

Hi Igor - seems like the issue is with the router rather than the SIM or mobile network.

Have you tried a full factory reset of the router? Can you double check the APN setting is correct also?

In reply to a post by Pheasant:

seems like the issue is with the router rather than the SIM or mobile network

I'm wondering if this is not a UK post, and there is an MTU issue.