Update 25/11/2021
I've decided to add Guardian into the build because it helps with shield gating or with frames with high shields (such as [Hildryn]) even if it's on a 30 second cooldown. Even if we put it as our first priority mod it won't mess up our Assault Mode to the point where our sentinel simply doesn't attack so that's good. Scratched a few lines in the guide since they are now redundant. The mod that got removed was Repair Kit, the passive +6hp/s heal was too little for it to be worth a mod slot, and you can pick up health orbs even when your frame is full hp to heal your sentinel, and you'll be spawning a lot of health orbs from Synth Deconstruct.

[Assault Mode] first so it can take priority. Guardian first priority to ensure that it always casts as soon as possible with Assault Mode second in priority otherwise it causes your sentinel to never restore your shields until you're out of combat.

You want your [Dethcube] to attack mobs so he can activate the Energy Generator mod as well as the Synth Deconstruct mod. Free energy and health orbs.

Enhanced Vitality, Metal Fiber and Synth Fiber for added tankiness on the little sentinel, Synth Fiber synergizes with Synth Deconstruct by giving your companion 100% armor on every health orb pick up and they'll be dropping from deconstruct.

Primed Regen and Repair Kit are fail safes to keep your sentinel up and running. Unlike animal companions like kubrows/kavats you can't revive your sentinels unless you die yourself and then revive, that's why we run Primed Regen for extra sentinel lives and Repair Kit for passive healing.
Health orbs that drop from Synth Deconstruct or in general also heal your companion so Repair Kit might be a bit redundant, if you want to drop a mod from the build, Repair Kit is probably the only mod to drop, it's our only flex slot.

Primed Animal Instinct and Vacuum are our standard loot mods, they are a must and essential QOL.

If you don't have Prime mods run their normal versions.

Run Verglas as your sentinel weapon ( it's [Nautilus] weapon) since it's the best sentinel weapon in the game currently since the nerf of gladiator mod set effect not working on warframe melees and the fall of Helios.

Remember that positioning of your mods matters, since the companions are programmed in such a way where they priorities which skills/mods they will cast. The priority is left to right top to bottom.
Now that you know that, Assault Mode must be first in priority. The rest of the special sentinel skills such as Primed Regen, Vacuum and Energy Generator don't matter where they are placed since they are passively active, the sentinel doesn't need to activate them himself.

Honorable mod mentions:

• Accelerated Deflection I really wish I would be able to fit this mod into the build since it allows for better shield gating, another way to make our companion more tanky, you could potentially drop Repair Kit for it but then you lose out on the passive healing, it's up to you. The way I put my formas you'll have enough mod capacity to play around between the two.
• Calculated Redirection please don't run this, Dethcube has base shields of 100 and armor doesn't apply to shields. Leave it as it is and prioritize on making his EHP higher by increasing his health and armor only. If you have only 100 shields you'll be able to shield gate much better.
• Guardian, Medi-Ray and Shield Charger these three mods are abilities that need to be cast by the sentinel, but since we are running Assault Mode as our top priority, these abilities will rarely cast. Shield Charger absolutely griefs your warframe if you're trying to shield gate with them trough Brief Respite, Decaying Dragon Key and augur mods. These three mods also have high cooldowns of 30 seconds (Medi Ray being 15sec). Medi-Ray can also be canceled if the sentinel decides to attack a nearby enemy while channeling it... As nice as these mods may seem on paper, they simply don't work in our build where we want our sentinel to spend most of his time attacking enemies.

*In case you don't know what shield gating is, it's a mechanic in the game where if your shields are broken you're given a short time of invulnerability. The first time your shields are broken you get 1.3sec of invulnerability (Hyldrin warframe gets 3sec, and everyone in Railjack missions gets 3sec) and every other time after that if your shields are broken you get 0.1 up to 1.3sec invulnerability depending on how much shields you've lost. So with lower max shield values and with higher shield [Regen], you might be able to permanently refresh that 1.3 seconds of invulnerability effectively making yourself tankier than the tankiest warframes!