I've followed the valuable advice on TM30 in past forums, but still have questions. 

Here's my specific situation:
I live in Bangkok in a condominium I own and have a Yellow Tabien Baan (and Pink ID Card).
However, I am registered with Immigration in another province since I used a visa agent for my 12-month retirement extension and subsequent 90-day reports.  
Since my financials are in order (balance of 800,000 baht in a local bank for over 12 months), I'd like to do the next extension by myself and without an agent.

If I'm not mistaken, I first need a TM30 using my condo's address.
I tried registering online (including uploading the necessary documents) but with no success (perhaps due to the agent having registered me in another province).
I'm considering doing it by post, since going to CW is not so convenient.
Your advice/suggestions would be gratefully appreciated.

use the mobile app. Super easy takes 5min to do.

screenshot in every step. when you are registering, the app automatically recognize your immigration area which is great. i always use the app

search 'section 38' you will see it. its like blue color and says 38 on it.

Thanks for the recommendation. I'll definitely look into the app.