Yes, the TP-Link RE550 (300 Mbps on 2.4 GHz and 1300 Mbps on 5 GHz) can be useful to improve a poor Wi-Fi connection, but it depends on a few factors.

When is it useful?
If the Wi-Fi signal is weak in some areas - The RE550 is an extender (repeater) that amplifies the existing signal, so it can help to cover areas where Wi-Fi reaches poorly.

If your main router supports 5 GHz - The 1300 Mbps band on 5 GHz is great for modern devices (smartphones, laptops, TVs) that support this frequency, offering faster and more stable performance.

If you have problems with speed but not latency - An extender improves coverage, but can introduce a slight delay (latency), so it is not ideal for competitive gaming.

When is it NOT the best solution?
If the problem is slow ISP (e.g. slow fibre) - The extender does not increase the speed of your internet connection, only the Wi-Fi coverage.

If your router is very old - If your original router does not support 5 GHz or has outdated Wi-Fi standards (e.g. Wi-Fi 4), the extender will not work wonders.

If you have interference or too many connected devices - In these cases, a new router or a mesh system might be better.

Alternatives to consider
Powerline with Wi-Fi (e.g. TP-Link AV600 or AV1000) - Useful if Wi-Fi does not reach well and you want to use the mains to extend your connection.

Mesh system (e.g. TP-Link Deco) - More expensive but offers even coverage without loss of performance.

Conclusion
If you have a weak Wi-Fi signal but a decent router, the RE550 can help, especially by exploiting the 5 GHz band. If, however, the problem is the Internet connection itself or an old router, consider upgrading your main router first.