As this is primarily a user to user forum, someone-else may chime in.
I am not sure how this works, so this is just an idea.
How many Serial Numbers did you get? What are the first 5 characters of each?
Do not, of course, post the full numbers here. Maybe the hardware Serial Number also Authorizes AmpliTube 4 ("AT4")?
Download from your User Area and install "AmpliTube Custom Shop". It installs three programs:
-- the AT4 standalone program and plug-ins,
-- the Custom Shop standalone program,
-- the Authorization Manager standalone program.
If on Windows, you might also download and install the iRig ASIO Driver for Windows.
Start the Authorization Manager program and try to Authorize AT4 using the iRig Serial number. Close the Manager.
Start the Custom Shop, login in you your User account, select the AmpliTube section, the select [Gear] then "Restore my Gear". This makes the AT4 gear models available. Close the Custom Shop.
Start up the AT4 program and in its Settings menu, open the Audio MIDI Set-up; there select your audio interface as the audio input and output devices. Details are in the AT4 User Manual. Check that you can load the AT4 gear models.
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IF that does not work then the best thing to do is raise a
Support Ticket, here for some one to one investigation and help (US (East Coast) Business hours, generally). Response time is usually within 2 working days, but in the current circumstances please allow some more time.
[#] But please remember that the same symptoms do not necessarily mean the same problem.
And, to help others who run into a similar problem, please let us know what you did to resolve the problem.