Are you using stock router from TMnet?

phone line? are you on streamyx?

if unifi, it should be fiber to ONU (huawei) -> TPLINK router -> Synology.

Also, the DDNS may not work due to Private IP:

TM Begins Migration For Select Unifi Customers From Public To Private IP

In that case better to just use QuickConnect without port forwarding. It will tunnel through QuickConnect instead of direct connection.

Tunnel through QuickConnect, the connection speed is capped at about 30mbps. But your DSL line upload speed is 1mbps only. You also no need to worry about risk with opening port.

Make sure to enable 2FA for all account as additional security. Only use not default admin account for NAS and backup management while use normal user account for read/write data.