Have you tried using something other than Gmail?
I've had the same problem with my Gmail accounts but it worked when I used a Yahoo email

I’ve tried several times to join the Pattaya forum with different Gmail accounts and have never received a confirmation email. Even reached out to a moderator but still nothing … 

Let's face it, the most influential business model in Thailand is the Ping Pong Show.

I'd definitely look for any alternative to the Landmark, as they are total muppets, or at least think we are. 
Looking at a quick trip either end of the month or early next.  Checked Landmark and saw that they have a "Book 2, Get 1 night FREE" deal for July, which was quite enticing. Selected the dates, selected "Premium Corner Room" @ THB4,733 per night, which is pretty much standard pricing for that room, but a great deal when you're getting a 3rd night free. I hit "Reserve Now" just to check that it's valid and lo and behold, it's a bit of a scam. 
Night 1 = THB7,100
Night 2 = THB7,100
Night 3 = FREE!

So all they've done is total up what they normally charged for three nights (THB4,733 x 3 = THB14,199)  and then divide that into two nights. Scumbags. 

I use gmail. I haven't had email notifications for years. I can only log on by clicking on old PM email links.