Very slow upload speeds on 4G with RUT360

I’ve been having an issue with upload speeds on the RUT360. Download speed is usually around 60Mbps, but upload is usually less than 0.2Mbps (with ocassionally 4-5Mbps, but that’s rare).
Putting the same SIM card in a different device (either a different router or a mobile phone) results in download speeds of 60-90Mbps and upload of 30-60Mbps.
I tried with 3 different external antennas, one of them raised by 3 meters.
I have clear line of sight to the mobile mast, which is 300m further on a non-metalic water tower.
I also request a new SIM card from the operator (Mobile Vikings, now owned by and using the network of Proximus in Belgium)

Speed test result :

Any ideas what migth be causing this ?

Adding mobile page as screenshot :

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