In terms of connectivity, the Galaxy A71 has Wi-Fi ac, Bluetooth 5.0 LE, A-GPS, and NFC. The phone has a Category 15 LTE modem with 3x carrier aggregation, and the global variant has the following LTE bands: 1/2/3/4/5/7/8/12/13/17/20/28/38/40/41/66.
https://www.androidcentral.com/samsung-galaxy-a71-review
This means that you’ll likely get higher speeds when using your phone, as opposed to a cheap router. Though not necessarily.
First stop is to see how DTAC performs at your place. DTAC offers the highest capped speed plan at 30Mbps, so it makes sense to include them. Thunder is a good offer, but it’s 199 baht for a test SIM with a month duration.