You should be able to get relevant paper form so all is not lost.
Write to the Inspector dealing with your affairs and give him full details of the sale, quoting both UTRs and he can arrange to issue an assessment, with info on how and when to pay. Payment can be made by cheque or electronically.
I am not convinced i would 100% agree with this being the best course of action - thankfully for digitally excluded individuals (or customers as hrmc would call you) there is a paper form that does the job - you just need to contact hmrc and assert that due to not being internet savy you do not feel safe being forced to submit anything online where you could be then comprimised with confidentail info - hmrc should without too much fuss issue you with relevant paper form to complete. If they resist issuing paper from IMHO they should be robustly challenged that they are forcing you down a route that could compromise your safety and you are not willing to do that. Sometiems you can get rogue advisor that will not do the right thing and might try and fob you off hopefully that is not the case - if you don't like what you here from advisor one i would say try once more to ask the smae question.
I was pleasantly surprised the one time i did request the paper form that at least that was promptly dispatched.
I think the problem with pawncobs route is that the formal mechanism should be used to submit details as far as i am aware (please correct me if i am wrong pawncob) and if you simply provide info by letter they may therefore default so writing back and saying that you should submit online - if you do go down pawcob's recommended route please directly highlight the fact that you do not feel safe submitting anything online so that if form needs completing you are formally requesting a paper form be sent out. Hopefully that will at least do the job of at least matching a request for apaper form via phonecall.
Note due to time limits incvolved i wouldnt trust that hmrc to repond to any written request so i would recommend getting through to hmrc helpline asap and requesting paper form on the basis you are not able to file digitally due to lack of intenrent saviness.
On an associated note i have just seen the following note ref filing online within 30 days - what a complete shambles hmrc are - and they have an afront to call you a customer when you have no ability to take your custom elsewhere who will treat you like a human being
Due to a temporary problem with the service, you cannot submit a return for the tax year 2021 to 2022 if one has already been sent for the tax year 2020 to 2021. If you submit a return, you may receive an incorrect calculation.