Cant create a working trunk on Cisco Router 881

[WorkingForUni]

Recently purchased this router for a little network at my university. All that i need it for is to connect two vlans that are on the same switch. I know i have a vrery little expirience with Cisco tech, but im sure i can do a router on a stick config with no problem.
The first thing i did was running a simulation in cisco packet tracer, to make sure that everything is working. Everything was pinging, no problems, no hiccups etc. Then i redrew the plan of the network along with some commands on a paper and started configuring. 
And thats when the problem has risen. I cant setup any sub-interface. I tried F1-4, G0-4 and only after that i found out that i can create F4.2 and F4.3 subinterfaces with proper incapsulation. After finding out that F4 is a WAN port ( which is weird design choice, if you ask me) i've completed the coding part and its still doesnt work. 
Did anyone happened to have a same problem as i ? 
A pic of network is for refference. 
Thanks in Advance 
-Mathew

[Karsten Iwen]

These devices work a little differently. You don't configure subinterfaces. Instead, you configure VLANs and VLAN interfaces with the needed IP config. Then you configure the physical interface to Trunk, and this interface becomes the connection for the router on a stick. Don't expect any good performance on this old device.

[WorkingForUni]

Thank you very much for the answering and explanation, but im not sure that i understand what you mean. Could you please give an example of how it should be configured ? 

[Karsten Iwen]

Quick and dirty ...

vlan 20
vlan 30

interface fast 1
 switchport mode trunk

interface vlan 20
 ip address 192.168.20.2 255.255.255.0
interface vlan 30
 ip address 192.168.30.2 255.255.255.0

[WorkingForUni]

If i understood you correctly, then i need to give those vlans the same ip that i give to my PC's as default gateway, right ? 
Already tried that , 100% lost packages =(

[Karsten Iwen]

Yes, that's correct. To test it, you can connect the PCs directly to the 881. That removes the switch as potential problems.

When the directly connected PCs can ping the routers IPs and each other (double check that the PCs don't have the host firewall enabled) then you can again add the switch and continue troubleshooting.

interface fast 2
 description PC-in-VLAN-20
 switchport mode access
 switchport access vlan 20
interface fast 3
 description PC-in-VLAN-30
 switchport mode access
 switchport access vlan 30

 For your other question:

Fast4 is meant to connect to the outside world like your ISP. The other ports are typically meant to connect end devices but you could also place a switch there.

[WorkingForUni]

Tried you method , but it doesnt seem to be working. The firewall( if i understood you correctly)  was off on both pc's , and i replicated both your messages in config. Unfortunatly , when trying to ping the gateway it says "General Failure" and if trying to ping another pc, then we get a good old " Connection Timeout" .
Im starting to think that my router may actually have some mechanical problems, here's the reason why the guy was selling it . 

[Karsten Iwen]

The shown config is incomplete, but the first half looks promising. I don't remember about the VLAN setup in the 881. Did you add the Vlans to the config?

vlan 20
vlan 30

 A hardware malfunction is also possible.

Another test:

1. Ping the PCs from the router
2. Do a "show arp" on the router
3. Do you see the mapping from the IP to the mac address on the router?

[balaji.bandi]

You mean to say on Cisco 881 Router you can not make sub-interfaces ?

can you post show version, show run completely

as per your drawing technically should work - what switch is this ?

[Joseph W. Doherty]

As @Karsten Iwen describes, the 800 series are "different".  Basically they were designed to directly host a few LAN devices.  I.e. only the WAN port is a routed port, the other ports are switch ports.

Further his second reply shows how you might trunk with another switch.

[WorkingForUni]

So the ports from 0 to 3 , are fot connecting switches , so the port 4 could give them a nat outside , right ? 

[Joseph W. Doherty]

BTW, what device(s) do you use in PT?

[WorkingForUni]

If by PT you mean the Packet tracer, then i used the 2960 siwtch and 4331 router (both in their default versions). 
My brain realized too late that there are other devices on the list

[Joseph W. Doherty]

Correct PT = Packet Tracer

Unfortunately, again, the 800 series is "different" from other ISRs.

Also, BTW, be cautious using PT for proof of concept.  PT often lacks many features found in the "real" device and the features supported might not be completely faithful to "real" device operation.