it will be worth while you asking them when you are there, if/how it would be possible to do online, for future TM30 submissions.
No doubt you'll be using Bangkok immigration in future for extensions etc and it's useful to have an accurate TM30 history. I had a couple of challenges because hotels I stayed in, outside Bangkok, had submitted a TM30 there. I needed to do another one, to cancel that one, when I returned to the condo.
I should add, this is normally done by your hotel, guesthouse, landlord and the process is very easy for them to do online. You just need to ask them for a copy of the acknowledgement they receive or screen shot