How to stop SYN FLOOR DoS on RV345?

[CraigConrad]

Hello! I have been under attack  that keeps dropping my network connection.  From the logs, it seems to be from a SYN-FLOOD.  The source is sometimes 162.125.6.14 and other times it is 162.125.9.14.

I have done everything I understand to do in the router's web interface, yet it continues.  The best way would be to chop the fingers off the jerk doing it, but I am hoping to find a more rational solution before I find him/her.   [UPDATE: now that it has been pointed out to me that these IP addresses belong to Dropbox, I am embarrassed for my tongue-in-cheek comment!]

I have enabled "Dos (Denial of Service)" in the Firewall Basic Settings, and even added these two IP addresses into the Firewall access rules to be "Denied" access altogether.  Nothing is working.

I would really, really appreciate any insight into this, as it is getting quite frustrating to have my connection down & up again all day long.

<warning>kernel: [69712.509622] FIREWALL SYN-FLOOD:IN=eth2 OUT= DST_MAC=28:ac:9e:0d:ca:54 SRC_MAC=:60:9c:9f:ce:f4:00 src=162.125.6.14 DST=xxx.xxx.xxx.xxx LEN=1500 TOS=0x08 PREC=0x20 TTL=59 ID=17904 DF PROTO=TCP SPT=443 DPT=55994 WINDOW=130 RES=0x00 ACK URGP=0 MARK=0xff00 

[Leo Laohoo]

Do you have any DropBox app?  

162.125.6.14 and 162.125.9.14 IP address originate from DropBox.

[CraigConrad]

Wow! Yes, I have Dropbox on my laptop, and have just established Cloud Sync (on my new Synology NAS) to my Dropbox account, and it is about 75% complete (after nearly 3 days) with its original synchronization.

If this is from that, I have questions.
1. Why would this look like an attack?
2. Why can't my router keep up with it (this is basically a home network on a 100Mb service)?
3. Why didn't it mess up Dropbox (which continued to sync) when I put these two addresses into my Firewall rules?

Thanks SO much!