@toac if you add second gateway, I assume it is connected to same power lines meaning that second one would have power issues at the same time. You simply need USV which can cover time without power, depending on how much devices and for which time you want to cover, but for simply tp-link network device quite any under $  100 will do the job which is cheaper than second router and ensures that you have no power issues. As you say you are in Africa, there is a lot of sun and many solar panels available for quite cheap, USV with solar panel is again still cheaper than new router.

  @btx assumption is the mother of all disasters  

Problem is that ER7206 is on other end of building on a separate UPS which cannot re-charge in time when we have multiple outages. The server room, where I would like to put backup router has a larger UPS, which can handle multiple outages. I am very familiar with solar, but it doesn't help me a night......I'm looking for possible solution to the question I asked, not an alternative energy solution.

  @toac maybe you should clarify on your question not to sound like if energy solution is what you need before claiming you did not ask for alternative energy solution, anyway, you still speak about outages and it is still about power usage. Sure you can buy X gateways, configure and use them, but I would still recommend to get an electrician even if few power cables have to be laid. According to what you wrote, you do not assume that solar can power together with your current solution and it is again up to you to count how much you need to cover 48 hours of outage, not just over night. I'll just let it go and leave you with assumptions, I still suggest to get competent electrician who will consult you on available solutions. Many ways lead to Rome.

  @btx thanks for input. I posted on an IT forum looking for a IT solution to get over a hopefully short term hurdle with consistent and long power outages. I am very involved in alternative energy with my own hybrid solar, inverter & battery solution and am very familiar with these options. Sadly this is not a viable option at the office where the problem exists. Hopefully someone on this IT forum can point me in a direction or advise if Omada SDN is limited to only working with one gateway device.

  @toac you are welcome. At no point I meant to use solar as main power supply, it was just one of many possible ways how charge a battery which powers your router. No, you are not limited to only one gateway, for each lan interface you can use different gateway (as long as you have available and free interface, managed switch is kinda something you would like to have in that setup). But omada will not ask you to if it is second gateway, you have to configure it by yourself. However, I still do not understand why you would require additional wan for a network which suffers on power issues, as if your router has no power, then other network devices must suffer on same issue. In my opinion your issue is power and if it is only about er7206 which uses few Watt, then a battery in many ways should resolve this issue. I fail to understand why and how do you want to resolve power issue with second or third or x'th gateway?

  @toac 

I've read somewhere about VRRP, that is a L3 switch function. It's kinda like

"So two Gateway devices will work together as a logical gateway. And all PCs can set this logical gateway as their default gateway. In this logical gateway, even one gateway is down, the other gateway will immediately take its place to ensure the whole network can continue working normally."

I'm not sure if I remember it correctly. But it seems tplink only has T2700 serious switch support this function. I can't find any Omada switches support it. 

Check T3700G User Guide page 239: