Sorry but I think you already new the answer after being out of the country for so long, and should really know when you go for your 90 day report so close to your extension ending they would be looking for an up to date TM30
I believe you will be due a 90 day report after 90 day ( Day of arrival counts as day 1) and apply for your extension 30 days before the one you have now runs out
If your in Roi Et your not a Million miles from Mukdahan, where you can nip over the Bridge to Laos and get a 30 day visa exempt when you reenter, have a day out then you can extend for another 30 back in Roi Et
Just by a cheap onward flight that will show Quatar in the UK you have an onward flight you can get them so cheap and throw it away, then go to immigration and extend for a further 30 day’s extension 1900 baht, make sure wher you are staying register a TM30 for you and get a copy you will need it when you go for your extension.