Сегодня сигнал слабее, чем вчера, поэтому мне трудно общаться онлайн.

Слабее is the comparative form of the adjective слабый (weak).

• слабый = weak (basic description)
• слабее = weaker (comparison)
  Because the sentence compares today with yesterday, Russian uses the comparative.

In Russian, one common way to express comparisons is:

• (comparative) + чем + (what you compare to)
  So слабее, чем вчера = weaker than yesterday.
  (Another option exists: вчерашнего after certain comparatives, but чем + вчера is the most straightforward and very common.)

• поэтому = therefore / so (it introduces the result)
• потому что = because (it introduces the cause)

This sentence is structured as: cause → поэтому → result
Today the signal is weaker than yesterday, therefore it’s hard for me…

Russian often uses an impersonal pattern for feelings/difficulty:

• мне трудно = it is difficult for me (literally to me it’s difficult)

So мне is dative because the difficulty is “directed” at/experienced by the person.

трудно is a predicative adverb (a “state word”) meaning it’s difficult, and it commonly combines with an infinitive:

• трудно + infinitive = it’s hard to do something
  So трудно общаться = it’s hard to communicate/chat.

Yes, word order is flexible. Your version is also correct; it just changes information flow:

• Original: cause first → result second (with поэтому)
• Your version: result first → cause second (with потому что)

Both are natural; the choice depends on what you want to emphasize first.