Buffalo + Router? Never thought they got into the networking market in the first place and would probably be the last name to look for in networking. They do offer some decent prices but durability isn't there for long term use on their products.
Firmware = software bugs
If your concern is security then don't allow remote access to keep all of the ports closed on the WAN side.
Consumer / popular options are usually riddled with bugs and issues though. Stepping up to a SMB type device is more stable because businesses won't put up with the antics on consumer devices.
@Tech9 has some Omada and other recommendations though. I would be looking for something that doesn't show up in the likes of Walmart / Best Buy / etc. Once you get away from the traditional retailers you get a better quality device typically. Also, those same retailers carry performance gear for power users like some of us around here that don't settle for generic heaps of junk.
If you want more control and security though you should consider making your own device out of common parts and configuring the firewall with a few simple entries.
Code:
-A INPUT -j PERMIT-IN
-A FORWARD -j PERMIT-FWD
-A OUTPUT -j PERMIT-OUT
-A PERMIT-FWD -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A PERMIT-FWD -m conntrack --ctstate NEW -j ACCEPT
-A PERMIT-FWD -j DROP
-A PERMIT-IN -i lo -j ACCEPT
-A PERMIT-IN -i br0 -j ACCEPT
-A PERMIT-IN -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A PERMIT-IN -j DROP
-A PERMIT-OUT -o lo -j ACCEPT
-A PERMIT-OUT -o br0 -j ACCEPT
-A PERMIT-OUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A PERMIT-OUT -m conntrack --ctstate NEW -j ACCEPT
-A PERMIT-OUT -j DROP
This is all I have in my filtering and unless it originates from the LAN it doesn't come back into the network from the outside. Most of the networking gear will use something similar but their macros clog up the entries with odd looking rules. A little bit of studying of homebrew setups goes a long way in protecting your own network and also not spending 2-3X the money on junk that's setup to fail when the warranty expires. With this approach you can take the OS drive to a new system if you want and update as frequently as you want by updating the kernel and underlying packages as needed. If you go with a decent CPU you can even get line rate VPN speeds using wire guard unlike the prepackaged off the shelf options that top out at 500mbps due to the lack of HW resources.
Using a WRT is a better idea in some cases compared to the OEM OS due to it being more current in terms of the packages being used. The issue remains tough that they tend to use older kernel versions though that might still be a security risk depending on how you setup the device. Using a router though from 2015 is going to be your biggest bottleneck when it comes to performance / speeds.
For under $200 you could turn a SFF PC into a router w/ Linux as the OS and then pick an AP for another $150 and get better results than consumer gear. It's not as simple as point and click with a pretty GUI but, once you set it up you don't need to do much to reconfigure things unless you want to. If you have an idea of what you want to do you just hit google for answers as someone else has probably posted the answer on a blog somewhere.