@Sunshine Thank you for the response.

The router is not from the ISP, it's been directly bought from a TP-Link retailer. It's not a pure ISP model, it's also advertised on the TP-Link conumer web site: https://www.tp-link.com/my/home-networking/wifi-router/nx510v/

Thanks for the answers, I agree with 1) router carrier aggregation support. I suspect U Mobile does not allow or support carrier aggregation in my area (Tanjung Tokong). And to save on DNB charges they steer traffic to their 4G network at night when it's less busy. Which means that I end up with 1 Mbps or less at times after midnight.

On 2) I also suspect this is a network originated traffic steering issue. I have logged a call with U Mobile a week ago but their support hasen't reached out yet. 

On 3) Yes, agreed. There is little information on SA deployments in Malaysia, possibly TM has one but I couldn't find anything about U Mobile so most likely it's NSA only.

I'll keep on waiting for U Mobile support to get back to me and then report back here.

  @MartinPEN I imported NX510v to india from Malyasia. Last month(Dec 2023)

It works with carrier aggregation (Jio 5G network). If it falls back to 4G automatically then select 5G only option. 
 

It supports both n28 & n78 bands and aggregation of them too. Here in india Jio is working on SA. Not on NSA which is anchored to 4G network.

only problem i observed till now is when we connect external anteena, internal antenna won't work to add aggregation. Initially it was working with aggregation of both n28 and n78 when external antenna connected but not now. 
 

however, it was must required device and very good one from TP-Link

@Sunshine please help us by modify firmware to support both internal and external antenna's to gain best signal

  @Lnreddy - thank you for your response.

Selecting 5G only option does not work in Malaysia. Unlike in India the operators are not ready with SA. "5G only option" will not work with NSA due to the 4G anchor.

N28 and N78 are both 5G bands. In Malaysia N28 is only used for the 4G anchor (signaling only no up/downlink - it's only 20 Mhz) therefore no inter-band aggregation on 5G. I am more concerned about 4G carrier aggregation as the signal on N78 is very weak. 

I still have no confirmation from TP-Link if the device support 4G carrier aggregation, if it does then I will have to find out from my provider why this is not working (I get only about 1 Mbps downlink on a 75% 4G signal).