Using Nighthawk M2 as router for my LAN (mesh?) - Looking for advice

[AB-NL]

Hi 😄

First the situation:
I am helping my parent to get better internet coverage in their house.
They live in a rural area in The Netherlands. There are no landlines so they will need to get their internet from 5G / Sim (Provides is KPN).

Until now they used a Huawei Sim-router E5186, close to a window.
Because the Huawei-Wifi-signal is terrible I connected a switch (similar to TP-Link TL-SG105PE) to this router and from the switch there is an UTP to the PC, but also an UTP going to an Engenius Access Point (I think Engenius ECB350, connected over PoE) to broadcast a better Wifi signal in the house. It is a different SSID than from the Huawei. Signal is better but still not great.

Because the Huawei is not the best option, the KPN Provider suggested a Mobile Router with a new Sim. That should give better strength and more flexibility. 

My parents ordered the Nighthawk M2. 
It took some hours but they got it to work! 🎉 so there is a good wifi-signal with the Nighthawk, if you are close to it.  The range it not very strong. 
But plugging in the UTP is not giving internet access through the Engenius-Wifi, like the Huawei used to do. 🙁

So now to the question:
- I want to use the Nighthawk as router, and by plugging in the LAN-UTP getting the Engenius to work for the house-Wifi. I don't want to put the Nighthawk in 'passthrough' because they want to be able to unplug the LAN-UTP and take the Nighthawk M2 on the road without a hassle.
- But even better: I want to get a Mesh-network behind the Nighthawk M2, preferably using the LAN-ethernet. Something like the the Netgear Nighthawk MK63 Mesh, Orbi-series or TP-Link Deco series (M4, P9, X50)

Simple questions:
- Is this possible without using passthrough? 
- Will it be relatively simple to connect and use a Mesh with its own SSID? (like Nighthawk MK63 / Deco M4)
- What can we do at this point to get the Engenius-Wifi to work?

Thank you!

[AB-NL]

Update:
It is working.
There was a problem with the managed Acces Point that (EnGenius) that had DHCP and the same gateway (possible cause) -> I took it down. Problem solved.

Now using it with a Devolo Magic 2 - Powerline Wifi. 

Still sometimes the devices (phone / laptop) will give an error 'No IP address found'. So somewhere it is not getting an IP.
But most of the time it is working...