Router – How to setup dual WAN & two LAN Networks correctly

We've just been switched from ISDN to SIP.
Since the guy who installed and configured the phone system does not have a lot of experience in setting up LAN networks, we need to do it ourselves.
My knowledge of setting up LAN networks is also quite basic.

Our current setup is the following:

Some of our configurations on our TP-Link router are the following:

WAN1
Connection Type: Static IP
IP: 5.10..
Subnet Mask: 255.255.255.0
Gateway: 5.10.. (IP of the ISP 1 modem)

WAN2
Connection Type: Dynamic IP
IP: 192.168.178.21
Subnet Mask: 255.255.255.0
Gateway: 192.168.178.1

System Mode: NAT

LAN IP: 192.168.111.1

DHCP: enabled

Bandwidth Control: disabled

Line Backup: no rules active

Rounting: Except for a few port forwardings nothing is configured

Router auto generated "Route Table":

Destination        Gateway         Flags   Logical Interface  Physical Interface  Metric  
------------------ --------------- ------- ------------------ ------------------- ------ 
0.0.0.0/0          192.168.178.1   GS      eth2               WAN2                 0       
0.0.0.0/0          5.10.*.129      GS      eth1               WAN1                 0       
5.10.*.0/24        N/A             C       eth1               WAN1                 0       
192.168.111.0/24   N/A             C       eth0               LAN                  0       
192.168.178.0/24   N/A             C       eth2               WAN2                 0       

Some of our configurations on our FRITZ!Box router are the following:

WAN: Auto setup through ISP

IP: 192.168.178.1

Subnet Mask: 255.255.255.0

DHCP: enabled

Routes & Port forwardings: none

What we want to achieve

The FRITZ!Box needs to be able to send and receive to/from the 192.168.111.* network, since on the clients there is a software installed to make and receive calls.

The clients should only be able to connect to the internet via ISP 1 only and therefore whit the IP 5.10...

What we noticed

The problem now is, that the TP-Link router uses the IP of ISP 2 most off the time. Since no load balancing or fallback is configured it should only use WAN1.

What also does not make any sense to me is, that although it seems that we access the internet via ISP 2 we do have the speed of ISP 1.

How can we achieve our goal?
If you need any further information, please let me know.

Thanks in advance!