Ammonia! Help! Please!

[HaveFishWillTravel]

I am not new. I should know how to do this. For some strange reason, I am still having trouble and I need immediate help.
Aquarium History:
-10 gallon live planted tank cycled and running since June 2020 w/ no serious problems.
Water Parameters on Average:
Ammonia = 0 Nitrates = 1 - 2.5 ppm
Nitrite = 0 ppm dKH = 3 pH = 6.8 - 7.2
Temp = 78 degrees F
-Fluval 107 Canister Filter (matrix carbon, bio-media rocks, ammonia pad, 2 blue eggshell sponge filters, 2 rectangular sponge filters)
Redundant Eheim Thermocontrol Heaters, Satellite LED+ Lighting
-Water changes 40% every 7 - 8 days using RO water. Treat new water with 5ml Prime. Don’t use fertilizer. Substrate is just rocks. Decor is rocks and a teak root that I scrub and rinse at each water change.
-7 Harlequin Rasboras,3 Glowlight Tetras, 2 Dwarf Cory Doras, 2 Dwarf Otto Catfish
-All fish have been healthy, no one has perished in 7 months.
Originally, this tank was in my garage, but it started to get too cold to manage it effectively as the temp in the garage dipped to 45 degrees F.
The temp in the tank started to fluctuate 75 - 79 degrees every 2 or 3 days and I had to remove and replace water to warm up the tank. This went on for about 3 weeks. I didn’t have anywhere to put the tank inside! Over the holidays, I built a table and brought them indoors. More stress and more water changes to get them inside because the tank was heavy. It took about 3 days to regulate the temp to 78 degrees. The only place I had to put them was in front of an eastern facing window. This was meant to be a temporary situation until I could figure out a better place for them. This is when the Ammonia started to rise. It started out as ~.25. Next 5 - 7 days, it had spiked to 2ppm. I siphoned the substrate, did not clean the filter, replaced the ammonia pad in the filter with a new one and started dosing.
1 dose Seachem Pristine (at water change)
7 days Sea hem Stability 2.5ml (mixed w/their water or at water change in new water)
7 days Prime 2.5ml (mixed with their water...).
Yesterday, I started to feed my little guys and the 3 Glowlight Tetras had perished.
I was devastated.
I tested and Ammonia = 4 ppm
It appeared that the Stability dosing/cycling had not worked.
What is it? what is going wrong?
The UV Light from the window?
Too many water changes plus bio-load?
I scoured the entire filter except for the bio-media, I siphoned the substrate (~10 gallons)...I made 20 gallons of RO water treated with Prime and did two subsequent water changes. Basically, flipped the water out twice last night.
I tested the water this morning:
Ammonia = 0 Nitrates = 0 pH = 6.8
I doubt this is going to last.
Should I dose/cycle with Stability again or try a different Bacteria?
I am providing some photos of the tank and of the products. I tried to be thorough in my explanation and I will appreciate any help or ideas that anyone can provide. Sometimes, it is the simplest things that we ourselves cannot see that is the problem. Thank you in advance.
Julie Lynn