So, now it says "4G" (AIS) on my mobile screen, I never asked for it, but there you go. Since this happened, my internet sucks. Any ideas? Single gateway maybe? Anyone else experiencing this?

4G is supposed to be significantly faster than 3G. However it's more sensitive to marginal reception conditions.

You may try browsing through the settings menu and see if you can find something that lets you select 3G/WCDMA instead of 4G/LTE as the connection mode for your SIM. Some phones have this option.

Then, there could be many other reasons also, like AIS having messed up the 3G->4G migration of their equipment in your area or more people pumping on the operator's links to the antennas in your area due to 4G, overloading them. Your phone could be at fault too (is it a good brand?), some cheap chinese phones have flaky 4G firmware.

dtac sims are available. make sure you specifically ask for a roaming one. only found out mine wasnt roaming when i was committed to going.