[dua] delay DUA registration to avoid transmission failure (#5893)

src/core/thread/dua_manager.cpp

@@ -321,7 +321,7 @@ void DuaManager::HandleNotifierEvents(Events aEvents)
         else if (mle.IsExpectedToBecomeRouter())
         {
             // Will check again in case the device decides to stay REED when jitter timeout expires.
-            UpdateRegistrationDelay(mle.GetRouterSelectionJitterTimeout() + 1);
+            UpdateRegistrationDelay(mle.GetRouterSelectionJitterTimeout() + kNewRouterRegistrationDelay + 1);
         }
 #endif
     }

src/core/thread/dua_manager.hpp

@@ -169,7 +169,7 @@ public:
 private:
     enum
     {
-        kNewRouterRegistrationDelay = 5, ///< Delay (in seconds) for waiting link establishment for a new Router.
+        kNewRouterRegistrationDelay = 3, ///< Delay (in seconds) for waiting link establishment for a new Router.
     };
 
 #if OPENTHREAD_CONFIG_DUA_ENABLE