@FinnieJP 

Hi,

May I know the model number of your main router?

Would the 2.4ghz and 5gzh lights turn off when the connection dropped?

Have you tried to change the wireless channel on the main router, like channel 11 for 2.4ghz and channel 40 for 5ghz?

If possible, please also try to enable the high speed mode on the RE450 v3, such as 2.4ghz to the main router, 5ghz to the clients to see whether it helped or not;

https://www.tp-link.com/en/support/faq/2479/

I noticed that you are not on the latest firmware and could you please update to the following firmware first:

https://static.tp-link.com/2021/202101/20210106/RE450_v3_201203.zip 

At the same time, I would follow up on your case by email, and please have a check of your email box later.

Thank you very much and wait for your reply.