Firewall on RT-AX86U blocking RTRR Server

[Pir]

Hi,
I am about to go crazy.... please help!

The IPs/ports are changed for security reasons.
I need to backup data from NAS1 to NAS2 on a scheduled basis (NAS: QNAP using HBS3 backup program). For that i have a RTRR Server running on NAS2 with the port 1234 and now need to set up the backup-push to NAS2 on NAS 1.

I have the following network (for specific reasons):

Router 1: RT-AX86U (IP outside something -> internal 10.1.10.10)
....- NAS1 - IP: 10.1.50.1
....- Router 2: RT-AX86U (10.1.10.20 -> internal 10.2.10.10)
...............- NAS2 - IP 10.2.50.1

I also have a route set up on router 1 for

IP 10.2.50.1
Mask 255.255.255.255
Gateway: 10.1.10.20

I TURNED OFF the firewall

When I go to NAS1, input the connection with IP 10.2.50.1 with port 1234 and test the connection. All works just fine.

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
So i turned the firewall ON. Of course it is dropping the packages.
with the message:

SRC=10.1.50.1
DST=10.2.50.1
PROTO=TCP
SPT= changing between 30000 and 46000
DPT=1234

so I set up the following (and tried many more)
-> firewall > generic > IP4 > incomming rules

rules activate: YES
incoming rules IP4
Source: 10.1.50.1
Port: 1234 (on other ocasions also 1:65535)
Prot: TCP

-> click apply

still cannot connect.

Then I read that the firewall might have issues and to use port-forwarding:
thus on top I went to Router 2 > WAN > Port forwarding

Service NAS
Ext Port: 1:65432
Int Port: 1234
Int. IP: 10.2.50.1
Prot: TCP
Source IP: empty (but tried also with 10.1.50.1).

add profile and enabled port forwarding

nothing.............
I have been fighting this for a day now, any ideas, what I am doing wrong?