I have been using WR841HP for around 3 years now, without any issues at all. Until recently, for the past few weeks, the connection became very unstable.

I have contacted the ISP, however, they claim that there isn't any issues on their side. So, I installed the original router that was provided to us and true enough, the connection is pretty stable and I am able to achieve 100mbps. However, even it's dual band, there is no 'wall penetration' technology thing, and the signal can't even get to upstairs. My repeater is of no help when there is no signal.

When I reinstalled WR841HP, the highest I can get is 40mbps (previously I am also able to achieve 100mbps). Yes, I have already updated the latest firmware.

Before, I have no issues getting wifi signal of 20mbps in my room located upstairs, until these few weeks, I am getting either fluctuating signal strength... For a few moments, I obtain 20mbps, most of the time between 1 to 5mbps. From time to time, it will get disconnected... The other rooms upstairs are getting pretty weak and unstable as well. I can summarise it in such a way that, 80% of the time 1 to 5mbps, wifi signal gets disconnected every one hour, and files that I am downloading indicates "network fail" and I will need to redownload them.

By changing channels (normally i assigned to auto), certain channels, I am not able to connect to the wifi at all...

Frustrated, I reset the router, and setup once again... The issue is the same...

I am getting very stable signal downstairs, but not upstairs.

My home package is 100mbps, I am not sure if my router is dying and I should get a like to like replacement, or upgrade to C58HP.

The general question is, is C58HP (or any long range router that anyone can recommend) better in terms of technology, range, etc compare to WR841HP. I am not expecting 100mbps in my room, even if it's 20 - 40mbps, I am alright. It's just that stability is what matters to me. What i understand is that, getting a higher speed router, does not mean that I will get faster internet connections. WR841HP is able to support up to 300mbps, which is more than enough.
My question is more in terms of stability and technology.

It is very frustrating when it happens a few times last two weeks ago, when I need to send out some documents urgently via email, I can't even send out files that is of a size of just like 2mb. In the end, I connect to my phone's hotspot.

Tried switching off both modem and router and restart it after a few minutes, results are the same.

Your kind advise is much appreciated. Been a few weeks now, I hope to put my wifi headache to rest as soon as possible.