I have no issue with my router & devices (mobile & pc) since day one Jan 2014, it is stable.
It is best to run some test on speed on various devices, and also leave it on for a period as you mention 'disappeared'.
In testing, my laptop did dis-connect or 'disappear' when i am on 2.4GHz. I had to manually de-activate the 2.4GHz and connect to 5GHz.
D-Link 865L have 2 bandwidth namely; 2.4GHz and 5GHz and each have its own SSID.
My router has pre-set SSID and password. Upon complete setup (Jan 2014), the Starhub guy gave a reference card on SSID & password and there is a sticker indicating the same. So i guess i better not change both SSID and Password.
I had upgrade from 200Mbps to 500Mbps in Jan 2015, when i was re-contract of mobile.
Let me explained on testing the router, as someone did said that he did update the firmware, in order to have better speed.
I did some testing to check on why my connection is still same as previous plan.
I need to do testing before any update firmware, if nec. For, say laptop 1 (32bit), i wired directly to the router.
I did a speed test using Starhub speed test. The test speed was around 350Mbps and at one glance when testing, it hit the 400Mbps.
Then i checked the properties, which indicate the max is 1Gbps. So in other word, there is nothing to do with the firmware, at all.
Next for, say laptop 2 (64bit) i use wifi, and using 5GHz.
I did the speed test, and the result is between 150Mbps to 200Mbps, no change, as i commonly use this for surfing.
So, i checked the properties which indicate the max is 300Mbps. So this is why i cannot go >200Mbps.
Even when i am inside the room, it is still able to be between 150Mbps and 200Mbps. I can accept this speed.
When i test on 2.4Ghz in my room, the result is very low around 50Mbps. So, i have to de-activate 2.4GHz on the laptop.
Then i re-set my Samsung S5 wifi to connect to 5GHz. Result speed is 250Mbps (LTE+).
When doing testing, all other devices that use wireless connection are deactivated. This is important, as it may cause the wifi to slow down.
It is better that the router is placed higher than any devices using wifi. The surrounding of the router should not being block.
Doing testing, it is best done near to the router.
My suggestion to you, is to do some testing. Both wireless and wired.
On the router 865, there should have a sticker indicating 2 SSID, for 2.4GHz and 5GHz.
On the mobile & pc, check which SSID is connected to. Switch between 2.4GHz and 5gHz to see which is better.
Again, it is better to do testing close to the router. Ensure your sifi setting is secured.
Read the post before yours (post by HPL).
When i switch on my laptop, it auto detect the 2.4GHz.
It did disconnect (disappear) by itself. So i had to manually de-activate 2.4GHz and use 5GHz.
This could resolved the issue about why your connection was disappear at times.