OMG, now you just know CAT5 is 100Mbps and CAT5e is 1000Mbps?

Since you mentioned your cable is cat5 and not cat5e, we can skip that LAN link speed check. Make sure you are using CAT5e cable from I240G to RGX4400 too. Same with LAN cable from RGX4400 to PC's Intel 1219-V

Not much we can do now.
Can you at least ensure CAT5e cable is being used to connect I240G and RGX4400 now?
Let start with fixing your 5Ghz signal first. First, set both 2.4ghz and 5.0ghz wifi channel width to 40Mhz and 80Mhz respectively.
Orient your RGX4400 vertically with the TM logo side facing the direction you wanna focus.
Speedtest your 5ghz smartphone again using TM speedtest server.

Max transmit power too.....

........-.-.......Just a quick check, how many 5Ghz hotspot detected on your area using the phone through wifi analyzer?

Not only you, but your phone unable to detect other 5Ghz hotspot.

Hmm.....can you change 5ghz channel to 56, 104, 124,136 and 149? Speedtest again with these different channels.
Check if changing to different 5ghz channel make any difference or not.
Previous horizontal orientation result:
Speed test on Samsung J7 Pro: 60.0 Mbps, upload: 93.5 Mbps ( 5.0 Ghz )

After vertical orientation result:
Download 89.97Mbps , Upload 57Mbps (5.0ghz)

Strange indeed....I wonder why the result kinda 'reversed' 

Aiyoyo.....try lah...change to different 5ghz channels......directly try channel 149........

.....I am starting to think maybe your phone has bad signal reception issue. Faulty phone antenna or hardware? Do you have any other phone with 802.11AC wifi chip? 
Impossible so close to the router and can't detect the 5ghz signal.......

That the strange part.
He posted a picture of the distance between his phone and router before 5ghz signal totally vanish here https://forum.lowyat.net/uploads/attach-74/...-1536766044.jpg

The distance is around 2 - 3 meters. Kinda impossible for signal loss to happen at that distance even for 5ghz.
Do you use any sort of phone cover? 
A metal in the phone cover will heavily attenuate or reflect the signal as well.

Try to remove the phone cover......

I notice there is a small fan close to the router. Possible the magnet from the fan causes interference to 5ghz signal? 

true, so he must test with another wifi and phones...

Indeed, we had exhausted every possible scenarios and solutions.
It is time to throw in the towel.....

Anyway, you will not be able to get 500Mbps with Asus USB3.0 AC55.

You will need 3x3:3 wifi adapter to match your planned DIR-878 purchase.

Set your 5GHz to 20MHz and channel 44.
Test and see if there is improvement.

Also, do you happen to have 2 SIM card for different country in your phone? Remove the foreign country SIM card if there is one

Dude, wifi is half-duplex which mean you only get around 150Mbps if you BTU connected CAT5e to Router.

Don't FANCY what's they write about...

then easy la...

Change everything including your router, cable, phone, laptop to get your dreamed turbo-speeds...   

Back to your old habits again. Read 1st page, 1st post, green text. Your 2.4GHz is congested or full of intereference. Others might be okay. Why do you keep asking? Get your own router already, and go on the 5GHz AC band. Get something good and stop asking.
I would recommend the TP-Link C9. Get it from OOKAS, and you will get the latest version, which is V5.

Sigh, start with your DIR-878 purchase.

Then speedtest 5Ghz 802.11AC with your J7 Pro. If speed is satisfactory and reach 200Mbps with your phone, then you can begin cable and adapter purchase.