I've stayed in different apartments in BKK and all of them had a very unstable internet connection. Half of time it's decent (~1MB/s) other times it's super slow and sometimes even barely usable. It's especially bad in the evening.

Is this normal? Any possible fix? Is getting a 4G hotspot the only option for situations where you need a somewhat reliable connection?

If you actually subscribe directly with an ISP then a BKK apartment Internet Connection is fine (except when it isn't during congestion or construction).

If you are on a SHARED Hardwired Internet Connection, or a SHARED Wireless WiFi Internet Connection then you should look for alternatives.

Those alternatives may be limited to a Mobile Internet 3G/4G connection if you are a short-time renter. Another alternatives might be an account with WiFi HotSpot Provider (if you are within range of a strong consistent signal)

If you are a long-term renter/leasee and will be in the same place for a year or more then look into AIS AirNet, TOT WiNet, or another direct ISP subscription connection.