Q:

Multi-WAN gigabit Router

(Yes, I’ve seen this question which is very similar, however my specs include the word gigabit. Also the only current answer to that question was written by someone that didn’t understand the meaning of “dual-WAN” and made an incorrect assumption about its meaning.)
I’m looking for a gigabit multi-WAN (or dual-WAN) Router for use in an automatic failover application.    I have a problematic cable connection which fails several times a month.  With my current level of home automation I can simply no longer accept that failure.
The cable modem is already a separate device (i.e. not an all-in-one combo device).  I have also already purchased a Netgear LB2120 4G cellular modem, which was supposed to also handle the failover routing (oddly the cable company blocks the device as a failover router, I can use it as a modem behind another router tho).  LB2120 also has no means of external configuration.  At the moment switching internet connections is as manual as walking in and swapping plugs, which doesn’t work if I’m not home.
I’ve also already tried purchasing, and recently returned to vendor, TP-Link TL-ER5120 v3 which should have solved the problem, but doesn’t because it has an odd firmware bug which seems to block ports on the local network.  (Specifically in testing port 3389–I suspect many others).  There is a bizarre long email chain with TP-link support where the initial entry level support associate validates my claim “in the lab”.  She passes the case to “senior level support” who consistently argues there is no problem found, and continues “coaching” me on setup. There are several really odd things that make me suspect about the “senior” techs understanding of his job, he insists: Static IP (not DHCP reservation) must be used, he also insists the client end (not the server end) be assigned the static IP, and finally he admits he is not using 5120 v3 but an older 6120 model that is “very similar” to mine.  He consistently tells me I don’t understand what to do, that I’m not following his instructions (even though I tried it his way and sent screenshots proving that I did).  There is also now another user in the TP-link public forum same complaint
Requirements include: 

Gigabit router, Multi or dual 
WAN with load balance/failover configurable options.
DHCP server, with reservations
Port Forwarding (i.e. opening an external port pointing to a specific internal IP and port).
No WiFi required (I already have a wired dual band AP in a centrally located position that provides excellent WiFi coverage).
not Enterprise level pricing (I can find this router many times over for $900+, however hardly any under $200, and only about one under $100).
~50 clients connected between a mix of wired and WiFi (but any gigabit router should handle that).
configureable remote access (I shouldn’t  actually need, but the fact the the Netgear LB-2120 has none exposes the potential need)

So to further reiterate, this router must have the ability to handle at least two internet (WAN) connections, one primary and one failover.
Edit 2020-05-30: (this question has gotten a lot of attention just lately). As indicated in the comments to the marked answer, two year ago I purchased the Ubiquiti ER-X.  I was very happy with the choice, however 2 years later, I’ve just recently upgraded to the ER-12.  The three elements that factored into the upgrade.  1) primary internet was upgraded to 1000Mbps (the ER-X didn’t perform well, even with hwnat enabled).  2) the router was running a VPN which was rarely used but slow for streaming video 3) the added hardwire ethernet jacks allowed me to eliminate a switch.   Configuration and scripts transferred almost without modifications.

A:

What you need is a product from one of the enterprise-level (or enterprise-like...) manufacturers. All of them have this in one form or the other, and they are usually not expansive at all (though most of them lack wireless, it is not required by you.) 
Ubiquiti EdgeRouter is probably the most known one of this genre. Then there's (as pointed out by @Jan Dorniak in the comments) MicroTik, and also UTT and other smaller brands.
But you need to be aware that all of these devices - given their enterprise-like status - while they are highly customizable and configurable, it is usually not as simple as the (dumb...) Asus or TP-Link and the likes, so there might be some learning curve to get to know them and be able to configure everything.

Ubiquiti has the perfectly capable EdgeRouter X for as low as $55. But if you are willing to pay up you can get the much more powerful (specs, but not necessarily needed) Edgerouter 8 Port Router for about $300. And in the middle there's the EdgeRouter POE if you can benefit of some PoE ports.
Here's a help article (and see links in the bottom) on how to configure the failover on Ubiquiti: https://help.ubnt.com/hc/en-us/articles/205145990
And, as mentioned, you can also look at UTT, for instance the UTT ER518 for just $54.

