Ten years old nightmare to make facetime calls to South America I live in Europe. Here I have a 200/100 MBPS fiber connection that works like heaven. Never drops, almost zero issues, an an uptime rate of almost 100% through the year. They have installed a super router here, capable of 433 MBPS. Everything works very well.
Then I have a family member in South America. She is using a DOCSYS connection. They say her connection speed is 400/200 MBPS but you know how crappy docsys technology is compared to fiber, but anyway, this is the problem.
She had an iPhone and iPad there, both configured with messages and facetime.
She is 70% computer literate.
Situations:

I facetime her. I listen it ringing here. She did not hear it ringing there.
I send her a message. The message stays clogged here and is not delivered.

Both problems happen if their devices are sitting there, without being used, for a while.
If I call her using a regular telefone, she goes to the devices, put them on airplane mode and them back to normal mode and now the calls and the messages work.
It is a nightmare.
The only thing I can think that she does is that she turns off messages and facetime over cellular when she arrives home (wifi).
She tells me that her phone and iPad always engage in cellular mode because they &quot;lose the wifi&quot; and because internet over cellular is expansive there, she turns cellular data off.
The telecom replaced her router 3 times and the current one is brand new, high speed router (2.4/5GHz).
What can be wrong? This problem is a nightmare.
Any ideas? Something in the router I can change?
Usually telco supplied WiFi routers are not of high quality. They tend to be low cost varieties with lots of other features bundled in that are of higher priority to the telco.
The best way to proceed is to disable the WiFi functionality in the supplied router. If possible, also disable the routing and NAT functionality to get a bridged connection only.
Then purchase your own router and WiFi equipment of higher quality. This also allows you to place multiple WiFi acccess point different places in the building. For example you typical want to have an access point on each floor (if it's a 2-floor house for example).
You can buy higher quality equipment from lots of different manufacturers. A popular choice for WiFi access points would be Ubiquity. If you do not have experience with WiFi equipment, I would recommened getting a professional to install said equipment. Around here it is very common to get offers on WiFi equipment including installation - I don't know if the same goes in that specific location in South America.