Accessing PC over VNC over Routerbourd with PPP

RouterOS Beginner Basics
1

Dear Mikrotik specialists.

I’m using a Mikrotik Routerboard 411U in order to setup a small Win7-PC and use it as a server for some small data exchange.
But for now I’m just trying to access the PC from outside via vnc.

The Router-Board has a SIM-Card and the PPP-Connection (A1net) is configured and running. The WAN IP is something like: 187.43.100.213
Spending quite some time with the wiki I managed to write a script which updates the WAN-IP to the dns-service dtdns.net
When I ping the ULR, the Domain-Name is resolved correctly, even though i get no packets back.
I configured NAT as well as DHCP, and the PC can access the Internet via the router(PPP).

Now, I tried to forward Port 5900 to the PC, which has a static DHCP-lease of 192.168.88.10
Using the manuals from the wiki, I try but fail. I think I configured everything right, yet I can’t seem to connect via VNC from outside.

I attached some screenshots from my IP/Firewall-Configuration.
In the Picture “2.jpg”, the “To-Address” and “To-Port” are configured to 192.168.88.10 port 5900:

I hope you can help. Thank you.
2.jpg
1.jpg

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Update:

This definitely seems to be some problem with outside access to the router.

When I use the routers 3G-Connection for internet access and try to ping the 3G-Interfaces public IP (or the Dynamic-DNS hostname) i get the full response.
I also can access the webfig-Configuration-Page using the Dynamic-DNS hostname.

However, when I use a altogether different internet connection, I get no ping-responses and can’t access the webfig…

Does anyone know the reason for that?

I also tried adding some specific filter rules to allow tcp/udp and icmp-traffic (see attachment)

I’m really at a loss for what to do…
1.png