Update:

I changed the device's operation mode to wireless router and connected it to my other modem to gain access to the internet.

Through that, I was able to update the firmware to  ‪1.3.0 0.9.1 v0001.0 Build 201207 Rel.56223n

Unfortunately, this did not change anything as the same issue still persists. Still cannot connect to 3G/4G and cannot read SMS

  @Sunshine thanks for the response.

I tried Advanced->Network->Internet page  and changing Network Search from Auto to Manual. It searched for around 30 seconds or so but I got this error message:

I also tried your other suggestion to insert the SIM into a phone and identify the 4G band using LTE discovery.

This is what I got.

  @Sunshine Hello. Sorry for the late reply, I got busy and was only able to look into this again today.

I am not entirely sure what happened but it started working again today! Using the same SIM (network #1 Smart) that I used last week, it connected to the internet automatically and speed is somewhat acceptable though connection is a bit spotty with disconnects here and there. I am writing this reply while connected to the same device.

As further test, I tried inserting a different sim of a different network (network #2 Dito). That network has better signal in my location but strangely, the device could not seem to connect to that one.

Here is a screenshot of LTE discovery on network #2:

At this point, I am glad that the device recognizes and can connect again to network #1 (Smart) but if you have any insights/suggestions on why the device cannot connect to network #2 (Dito), that would be very appreciated. I already tested network #2 on my phone and it has no issues, also tried hotspot and connecting my laptop, no issues. And also, I already got network #2 with this exact same SIM to work on this TPlink device a few months before.

Thanks!