more updates:

Ja! activaton (to default mobile data plan) - done (24 hours)

top up (via phone only!) - done (approx 5+ hours before you see credit online)

now trying to activate mobile data flat rate plan (15 / month for 3 Gb) online.
Note says that condition to switch is that you have enough credit on your account/card (20 euros now, 5 part of the package, 15 top up). When press OK button - nothing happens.

Initial opinion about Ja! - saying politely ... could be better ...
Not politely <CENSORED>

just to avoid confusion - Ja! activation and online access uses Congstar web-site, BUT in it's own area. Congstar prepaid user login/activation and Ja! are different

You can submit those USSD codes from your computer if you run an appropriate dashboard like mwconn or MDMA or if you use a modified version of Mobile Partner with USSD activated (Mobile Partner supports USSD codes but the plugin is hidden by default).

The customer portal does not display your balance in realtime. Check credit by *100# for realtime information.

Try another browser.

wow. It's all working ;-)

details - good advise about changing browser (didn't work correctly with Chrome)

APN should be set to same value as for any t-mobile internet device internet.t-mobile
login and password fields to be left empty

modem unlocking works for tchibo modem

thank you very much!

by the way - it looks like T-Mobile(congsar/Ja!) is blocking Skype.

Skype login is OK, chats are OK, but every time i start a conversation ... in 20-30 seconds half of frames are missing and we can not hear each other ...
Tried to take laptop far away from the building, tried some other places, parks, etc - same result. Even skype test doesn't work (same issues).

Playing with incomming port number (80/443) doesn't help...

Ehhhhhhh ....

In T-Mobile network VoIP usage is prohibited (if you dont buy a special VoIP package). I dont know if they scan the data and actively disturb the connection.

Quote: "VoIP, BlackBerry-Nutzung, Peer-to-Peer, Instant Messaging sind nicht Gegenstand des Vertrages." - Voip, ...., are not part of the contract

T-Mobile and Vodafone both block Skype and other VoIP services by actively dropping or delaying packets. E-Plus and O2 both allow Skype and VoIP, however given E-Plus' frequent network congestions O2 (including Lidl and Fonic) is the only reliable option for mobile Skype and VoIP in Germany.

Regarding inquisitors post - you can sign up for a free account at sipgate.de, they deliver a free VPN tunnel app together with their service which will enable you to use VoIP also on T-Mobile/Vodafone. (sipgate team - Hilfe)

sounds interesting ...
I'll post over here a result of trying it via VPN channel
Just need to go through sipgate.de terms, not sure why do they do it absolutely for free ... ??? what's the trick? :-)

thanks, guys, your assistance is much appreciated!

just found an article with a similar advise
What to Do if Skype is Blocked Where You Travel (and Why We Should Care) | almost fearlessalmost fearless