[technical inquiry] Fiber line capacity and router capacity.

Setup: I live in Jordan and have fiber internet from a company called orange. The modem/router given is a Nokia device (Model 140W-C).
Problem: I repeatedly lose connection suddenly and the tech support says they aren't detecting a "loss of connection/service" on their side, but are suggesting that:my TP-Link extender is causing compatibility issues and that there are generally too many devices connected.
Info/Troubleshooting: a) disconnected the TP-Link extender but problem didn't go away
b) company's initial offer is just an extender i buy at their store, assuming that this should be more compatible.honestly i'm not buying the story, this is a mere assumption. no guarantee.
MY Questions:
Q1: how do i actually evaluate whether i'm having "too many" devices on the router? (i accessed the list but i don't know what's ok and what's too much!)
Q2: if i think of improving my setup and maybe changing the system by having a MORE CAPABLE main device and then maybe two or three sub-devices (multi-story house, two families) . Then, how would i decide whether the single fiber internet subscription can handle that load ?---------------------------------------
usually connected devices:
1 laptop via ethernet. (gives 34 Mbps on ookla speed test)
1 TV via 5 ghz wifi
4 phones via 5 ghz wifi
6 phones/tablets via 2.4 ghz wifi.