Useful information to include:

• Minecraft version: 1.15.2
• CC: Tweaked version: 1.87.1
• Mekanism version: 9.10.5.418
• Detailed reproduction steps: Just place any energy cell from Mekanism (Elite Energy Cube) for example and try to attach modem (wired/wireless) to it. Nothing happens.

It might be a transparent block. Full block modems exist to fix this, just place the small modem in your crafting grid on its own to convert it. You can convert back the same way.

Also, unless you have plethora or mekanism provides its own integration, the cell will not be seen as a peripheral.

Problem is, Plethora is not updated to the latest game version.
Also, just tried to use full block modem, and as you have said, Mekanism does not provide integration out of the box. So the only way is to wait for Plethora upgrade.

feel free to contibute

In previous Minecraft versions (1.12.2 and before), integration was provided by Mekanism. However, it appears that has been dropped in recent versions.

I (or someone else) can have a look at adding it back once #447 is merged - that's rather the blocker for any other mods integrating with CC:T right now.