How to use Electrum for Android as cold storage?

[Lumpi101]

I have created an offline wallet on an old phone.

I can create an unsigned transaction with the corresponding watch-only wallet and show it as a qr-code so it can be read by the offline phone.
With the offline wallet I go to "send" and read the qr-code (the unsigned tx). Then I can see the unsigned tx on the offline phone (as an info) but i cannot sign it or do anything useful with it (despite showing it as a qr-code again)...

So how can I sign the tx and give the signed tx back to the watch-only wallet in order to transmit it to the network?

[unamis76]

Maybe you imported a watch only address and not your MPK like this person here? Please confirm.

[Abdussamad]

I have created an offline wallet on an old phone.

I can create an unsigned transaction with the corresponding watch-only wallet and show it as a qr-code so it can be read by the offline phone.
With the offline wallet I go to "send" and read the qr-code (the unsigned tx). Then I can see the unsigned tx on the offline phone (as an info) but i cannot sign it or do anything useful with it (despite showing it as a qr-code again)...

You have to use the load transaction option not the send option. I'm not familiar with electrum for android so all I can suggest is that you look for a load transaction option.

[Lumpi101]

@unamis Indeed, I imported only a BTC address because I thought it would be sufficient. With the imported MPK there is the sign-button which I searched before. Now, everything works fine.
For all who want to know how it's done: via the link of unamis is a good explanation.