@Matt76 

1. Should I bridge my ISP provider's router or not?

You should not. Keep ISP router as is, just turn its WiFi off after you configured Deco mesh.

2. Should I use the X90 in router or AP mode after bridging my ISP's router?

You should use X90 in AP mode. Deco Access Point mode delivers maximum performance, because that mode removes all and any traffic throttling and content filtering features in Deco.

Note that you can only use X90 in AP mode if you did not bridge ISP router. So, see answer to #1.

3. Should I wire both X90s?

Absolutely! Also, with X90 in AP mode you can wire X90s any way you want, as long as they connect to your home network. When Deco runs in router mode, there are restrictions in how you can wire Deco units. See General questions about Ethernet Backhaul feature on your Deco. If you want less headaches with wiring X90s, see answer to #2.

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As you may have guessed, I run my Deco mesh, consisting of 4 units, in Access Point mode.

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Feel free to recommend the best way to set everything up!

You have WiFi mesh from Deco. For WiFi mesh, in general, placement of WiFi nodes is important.

You have two nodes, two X90s. If your apartment is single floor, start with placing both X90s in opposite corners of the floor. If you find WiFi signal in the middle of the floor too weak or too slow for your taste, move both or one of units closer. Keep moving them closer to each other till you have perfect coverage everywhere. Very important to keep them wired, so if you have to move them - move to where Ethernet connection is. 

If you can't get perfect coverage everywhere, even after trying different places for X90s, consider buying one more X90 for the middle of the floor, with original two units at both sides of the floor.