External Linear PSU for Router - Voltage/Amps?

Match the voltage, excess Amps is OK.  For instance: 12V, 8 A is just fine.

The reality is a lot of nominally 12V linear supplies may provide slightly elevated voltages when unloaded, so you may read say 13.8V on a multimeter.  That's fine, and even 15V might be OK.

So long as the voltage is reasonable, the Amps drawn will only be what the device needs, so even if you had a 10A supply, the device will still draw < 3.5A.

If you use a power conditioner, keep routers and switches outside of it so they don't pollute the clean side of the power.

You have voltage correct, voltage must match. Amps listed on component's power supply is generally a minimum of what's needed, so you need lps rated for at least 3.5A, Absolutely no harm in lps supplying over 3.5a, in fact the more the better, helps in keeping heat down on lps because lps working at less than maximum capacity. The 4amp TeraDak would work fine, only issue I have is routers may be working 24/7 drawing higher amps than some other equipment. For max reliability over long term I use lps with higher amp output with my Netgear router, I'd go for 5 to 7 amps output, 12a you're looking at just fine, just spending more money than need to.

@erik_squires ​​@sns    Words of wisdom from both of you and many thanks.  I will search for a 12V linear power supply with amp output in the 5-7 range.