Is it realistic to stay under $30?

Yes, provided you avoid alcohol, domestic flights, and private hotel rooms. It is a 'backpacker' lifestyle, not a vacation lifestyle.

Should I bring an ATM card?

Yes, but be aware that Thai ATMs charge a flat fee of 220 THB ($6) per withdrawal. Withdraw the maximum amount allowed each time to minimize the impact of this fee.

How to survive Thailand on $30 a day

To stick to a $30 daily budget in Thailand, you must stick to street food, use public transit like buses and trains instead of flights, and book dormitory beds in social hostels. Avoid alcohol and guided tours, opting instead for walking tours and free public parks.

Prioritize street food over restaurants. Stick to night markets and roadside stalls. A plate of Pad Thai or Khao Pad (fried rice) at a local market costs between 40-60 THB ($1.20-$1.80). Avoid sit-down restaurants with English menus, as prices there are often triple.
Use local transport exclusively. Avoid taxis and Grab (rideshare). Use the BTS/MRT in Bangkok, city buses, and 3rd-class trains for long-distance travel. A 3rd-class train ticket from Bangkok to Chiang Mai can be as low as 230 THB ($6.50).
Book dormitory beds in advance. Use hostel platforms to find dorm beds for $7-$10 per night. Filter by 'guest rating' to ensure you aren't sacrificing security for price. Always carry a padlock.
Limit alcohol and nightlife. A single beer in a bar can cost $3-$5, which is 15% of your daily budget. Stick to bottled water (buy large jugs at 7-Eleven to refill a reusable bottle) and avoid cocktails.

Is it realistic to stay under $30?: Yes, provided you avoid alcohol, domestic flights, and private hotel rooms. It is a 'backpacker' lifestyle, not a vacation lifestyle.
Should I bring an ATM card?: Yes, but be aware that Thai ATMs charge a flat fee of 220 THB ($6) per withdrawal. Withdraw the maximum amount allowed each time to minimize the impact of this fee.